The motion of billiards in ellipses
Transcript of The motion of billiards in ellipses
The motion of billiards in ellipses
Hellmuth Stachel
stacheldmgtuwienacat mdash httpswwwgeometrietuwienacatstachel
Differential Geometry Seminar
Dec 9 2020 Vienna University of Technology
Table of contents
1 Metric properties of confocal conics
2 Confocal conics and billiards
3 Periodic N-sided billiards
4 A switch to Analysis
Acknowledgement
Dan Reznik Data Science Consulting Rio de Janeiro Brazil
Ronaldo Garcia Universidade Federal de Goiaacutes Goiania Brazil
Dec 9 2020 Differential Geometry Seminar TU Wien 134
1 Metric properties of confocal conics
F1 F2
P
e
The optical property of ellipses is well known and also the equivalence
equal angles lArrrArr F1P + F2P = const lArrrArr P isin e
There is a generalization
Dec 9 2020 Differential Geometry Seminar TU Wien 234
1 Metric properties of confocal conics
e
c
P
Q1 Q2
If any ray is reflected in a conic e thenthe incoming and the outgoing ray aretangent to the same confocal conic c called caustic
F1 F2
P
e
c
Q1Q2
Charles Graves (1812-1899) bishop ofLimerick and mathematician
The locus of point P used to pull thestring taut around c is a confocal ellipse e
De = PQ1 + PQ2 minusQ1Q2 = const
Dec 9 2020 Differential Geometry Seminar TU Wien 334
1 Metric properties of confocal conics
c
e
c
e
Billiards in an ellipse e are always tangent to a confocal ellipse or hyperbola
If one billiard closes after N reflections then all billiards close independent of the initialpoint on c (Poncelet porism) and all these closed loops have the same length
Dec 9 2020 Differential Geometry Seminar TU Wien 434
1 Metric properties of confocal conics
For centuries billiards attracted the attention of mathe-maticians beginning with J-V Poncelet and A Cayley
S Tabachnikov Geometry and BilliardsAmerican Mathematical Society 2005
Recently Dan Reznik revitalized the interest bycomputer animations showing the variation of periodicbilliards He identified 40 invariants eg a constantsum of Cosines of interior angles
Sum of square altitudes to N-periodic tangents is invariant
The two types of self-intersected 7-periodics in the Elliptic Billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 534
1 Metric properties of confocal conics
A family of confocal central conics
x2
a2 + k+y 2
b2 + k= 1
k isin R minusa2minusb2 sends througheach point P outside the axesone ellipse and one orthogonallyintersecting hyperbola
The parameters (ke kh) define theelliptic coordinates of P with
minusa2 lt kh lt minusb2 lt ke
P
F1 F2M
We specify the caustic c (semiaxes ac bc) as k = 0 and the ellipse e with semiaxesae be as k = ke =rArr ke = a
2e minus a2c = b2e minus b2c gt 0
Dec 9 2020 Differential Geometry Seminar TU Wien 634
1 Metric properties of confocal conics
From given (ke kh) follows
x2 =(a2 + ke)(a
2 + kh)
d2
y 2 = minus(b2 + ke)(b
2 + kh)
d2
Conversely P = (ae cos t be sin t)with tangent vector
te(t) = (minusae sin t be cos t)and tc(t)=(minusac sin t bc cos t) yield
P
F1 F2M
kh(t) =minus (a2c sin2 t + b2c cos2 t) = minustc(t)2 = minuste(t)2 + ke
Points on confocal ellipses e and c with the same parameter t have the same coordinatekh ie they belong to the same confocal hyperbola
Dec 9 2020 Differential Geometry Seminar TU Wien 734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12Given P = (ae cos t be sin t)the tangents from P to c (k = 0)include the angle θ(t) where
sin2θ(t)
2=
kete(t)2
tanθ(t)
2= plusmn
radicke
tc(t)
cos θ = 1minus 2kete(t)2
=kh(t) + kekh(t)minus ke
Dec 9 2020 Differential Geometry Seminar TU Wien 834
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
Table of contents
1 Metric properties of confocal conics
2 Confocal conics and billiards
3 Periodic N-sided billiards
4 A switch to Analysis
Acknowledgement
Dan Reznik Data Science Consulting Rio de Janeiro Brazil
Ronaldo Garcia Universidade Federal de Goiaacutes Goiania Brazil
Dec 9 2020 Differential Geometry Seminar TU Wien 134
1 Metric properties of confocal conics
F1 F2
P
e
The optical property of ellipses is well known and also the equivalence
equal angles lArrrArr F1P + F2P = const lArrrArr P isin e
There is a generalization
Dec 9 2020 Differential Geometry Seminar TU Wien 234
1 Metric properties of confocal conics
e
c
P
Q1 Q2
If any ray is reflected in a conic e thenthe incoming and the outgoing ray aretangent to the same confocal conic c called caustic
F1 F2
P
e
c
Q1Q2
Charles Graves (1812-1899) bishop ofLimerick and mathematician
The locus of point P used to pull thestring taut around c is a confocal ellipse e
De = PQ1 + PQ2 minusQ1Q2 = const
Dec 9 2020 Differential Geometry Seminar TU Wien 334
1 Metric properties of confocal conics
c
e
c
e
Billiards in an ellipse e are always tangent to a confocal ellipse or hyperbola
If one billiard closes after N reflections then all billiards close independent of the initialpoint on c (Poncelet porism) and all these closed loops have the same length
Dec 9 2020 Differential Geometry Seminar TU Wien 434
1 Metric properties of confocal conics
For centuries billiards attracted the attention of mathe-maticians beginning with J-V Poncelet and A Cayley
S Tabachnikov Geometry and BilliardsAmerican Mathematical Society 2005
Recently Dan Reznik revitalized the interest bycomputer animations showing the variation of periodicbilliards He identified 40 invariants eg a constantsum of Cosines of interior angles
Sum of square altitudes to N-periodic tangents is invariant
The two types of self-intersected 7-periodics in the Elliptic Billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 534
1 Metric properties of confocal conics
A family of confocal central conics
x2
a2 + k+y 2
b2 + k= 1
k isin R minusa2minusb2 sends througheach point P outside the axesone ellipse and one orthogonallyintersecting hyperbola
The parameters (ke kh) define theelliptic coordinates of P with
minusa2 lt kh lt minusb2 lt ke
P
F1 F2M
We specify the caustic c (semiaxes ac bc) as k = 0 and the ellipse e with semiaxesae be as k = ke =rArr ke = a
2e minus a2c = b2e minus b2c gt 0
Dec 9 2020 Differential Geometry Seminar TU Wien 634
1 Metric properties of confocal conics
From given (ke kh) follows
x2 =(a2 + ke)(a
2 + kh)
d2
y 2 = minus(b2 + ke)(b
2 + kh)
d2
Conversely P = (ae cos t be sin t)with tangent vector
te(t) = (minusae sin t be cos t)and tc(t)=(minusac sin t bc cos t) yield
P
F1 F2M
kh(t) =minus (a2c sin2 t + b2c cos2 t) = minustc(t)2 = minuste(t)2 + ke
Points on confocal ellipses e and c with the same parameter t have the same coordinatekh ie they belong to the same confocal hyperbola
Dec 9 2020 Differential Geometry Seminar TU Wien 734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12Given P = (ae cos t be sin t)the tangents from P to c (k = 0)include the angle θ(t) where
sin2θ(t)
2=
kete(t)2
tanθ(t)
2= plusmn
radicke
tc(t)
cos θ = 1minus 2kete(t)2
=kh(t) + kekh(t)minus ke
Dec 9 2020 Differential Geometry Seminar TU Wien 834
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
F1 F2
P
e
The optical property of ellipses is well known and also the equivalence
equal angles lArrrArr F1P + F2P = const lArrrArr P isin e
There is a generalization
Dec 9 2020 Differential Geometry Seminar TU Wien 234
1 Metric properties of confocal conics
e
c
P
Q1 Q2
If any ray is reflected in a conic e thenthe incoming and the outgoing ray aretangent to the same confocal conic c called caustic
F1 F2
P
e
c
Q1Q2
Charles Graves (1812-1899) bishop ofLimerick and mathematician
The locus of point P used to pull thestring taut around c is a confocal ellipse e
De = PQ1 + PQ2 minusQ1Q2 = const
Dec 9 2020 Differential Geometry Seminar TU Wien 334
1 Metric properties of confocal conics
c
e
c
e
Billiards in an ellipse e are always tangent to a confocal ellipse or hyperbola
If one billiard closes after N reflections then all billiards close independent of the initialpoint on c (Poncelet porism) and all these closed loops have the same length
Dec 9 2020 Differential Geometry Seminar TU Wien 434
1 Metric properties of confocal conics
For centuries billiards attracted the attention of mathe-maticians beginning with J-V Poncelet and A Cayley
S Tabachnikov Geometry and BilliardsAmerican Mathematical Society 2005
Recently Dan Reznik revitalized the interest bycomputer animations showing the variation of periodicbilliards He identified 40 invariants eg a constantsum of Cosines of interior angles
Sum of square altitudes to N-periodic tangents is invariant
The two types of self-intersected 7-periodics in the Elliptic Billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 534
1 Metric properties of confocal conics
A family of confocal central conics
x2
a2 + k+y 2
b2 + k= 1
k isin R minusa2minusb2 sends througheach point P outside the axesone ellipse and one orthogonallyintersecting hyperbola
The parameters (ke kh) define theelliptic coordinates of P with
minusa2 lt kh lt minusb2 lt ke
P
F1 F2M
We specify the caustic c (semiaxes ac bc) as k = 0 and the ellipse e with semiaxesae be as k = ke =rArr ke = a
2e minus a2c = b2e minus b2c gt 0
Dec 9 2020 Differential Geometry Seminar TU Wien 634
1 Metric properties of confocal conics
From given (ke kh) follows
x2 =(a2 + ke)(a
2 + kh)
d2
y 2 = minus(b2 + ke)(b
2 + kh)
d2
Conversely P = (ae cos t be sin t)with tangent vector
te(t) = (minusae sin t be cos t)and tc(t)=(minusac sin t bc cos t) yield
P
F1 F2M
kh(t) =minus (a2c sin2 t + b2c cos2 t) = minustc(t)2 = minuste(t)2 + ke
Points on confocal ellipses e and c with the same parameter t have the same coordinatekh ie they belong to the same confocal hyperbola
Dec 9 2020 Differential Geometry Seminar TU Wien 734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12Given P = (ae cos t be sin t)the tangents from P to c (k = 0)include the angle θ(t) where
sin2θ(t)
2=
kete(t)2
tanθ(t)
2= plusmn
radicke
tc(t)
cos θ = 1minus 2kete(t)2
=kh(t) + kekh(t)minus ke
Dec 9 2020 Differential Geometry Seminar TU Wien 834
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
e
c
P
Q1 Q2
If any ray is reflected in a conic e thenthe incoming and the outgoing ray aretangent to the same confocal conic c called caustic
F1 F2
P
e
c
Q1Q2
Charles Graves (1812-1899) bishop ofLimerick and mathematician
The locus of point P used to pull thestring taut around c is a confocal ellipse e
De = PQ1 + PQ2 minusQ1Q2 = const
Dec 9 2020 Differential Geometry Seminar TU Wien 334
1 Metric properties of confocal conics
c
e
c
e
Billiards in an ellipse e are always tangent to a confocal ellipse or hyperbola
If one billiard closes after N reflections then all billiards close independent of the initialpoint on c (Poncelet porism) and all these closed loops have the same length
Dec 9 2020 Differential Geometry Seminar TU Wien 434
1 Metric properties of confocal conics
For centuries billiards attracted the attention of mathe-maticians beginning with J-V Poncelet and A Cayley
S Tabachnikov Geometry and BilliardsAmerican Mathematical Society 2005
Recently Dan Reznik revitalized the interest bycomputer animations showing the variation of periodicbilliards He identified 40 invariants eg a constantsum of Cosines of interior angles
Sum of square altitudes to N-periodic tangents is invariant
The two types of self-intersected 7-periodics in the Elliptic Billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 534
1 Metric properties of confocal conics
A family of confocal central conics
x2
a2 + k+y 2
b2 + k= 1
k isin R minusa2minusb2 sends througheach point P outside the axesone ellipse and one orthogonallyintersecting hyperbola
The parameters (ke kh) define theelliptic coordinates of P with
minusa2 lt kh lt minusb2 lt ke
P
F1 F2M
We specify the caustic c (semiaxes ac bc) as k = 0 and the ellipse e with semiaxesae be as k = ke =rArr ke = a
2e minus a2c = b2e minus b2c gt 0
Dec 9 2020 Differential Geometry Seminar TU Wien 634
1 Metric properties of confocal conics
From given (ke kh) follows
x2 =(a2 + ke)(a
2 + kh)
d2
y 2 = minus(b2 + ke)(b
2 + kh)
d2
Conversely P = (ae cos t be sin t)with tangent vector
te(t) = (minusae sin t be cos t)and tc(t)=(minusac sin t bc cos t) yield
P
F1 F2M
kh(t) =minus (a2c sin2 t + b2c cos2 t) = minustc(t)2 = minuste(t)2 + ke
Points on confocal ellipses e and c with the same parameter t have the same coordinatekh ie they belong to the same confocal hyperbola
Dec 9 2020 Differential Geometry Seminar TU Wien 734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12Given P = (ae cos t be sin t)the tangents from P to c (k = 0)include the angle θ(t) where
sin2θ(t)
2=
kete(t)2
tanθ(t)
2= plusmn
radicke
tc(t)
cos θ = 1minus 2kete(t)2
=kh(t) + kekh(t)minus ke
Dec 9 2020 Differential Geometry Seminar TU Wien 834
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
c
e
c
e
Billiards in an ellipse e are always tangent to a confocal ellipse or hyperbola
If one billiard closes after N reflections then all billiards close independent of the initialpoint on c (Poncelet porism) and all these closed loops have the same length
Dec 9 2020 Differential Geometry Seminar TU Wien 434
1 Metric properties of confocal conics
For centuries billiards attracted the attention of mathe-maticians beginning with J-V Poncelet and A Cayley
S Tabachnikov Geometry and BilliardsAmerican Mathematical Society 2005
Recently Dan Reznik revitalized the interest bycomputer animations showing the variation of periodicbilliards He identified 40 invariants eg a constantsum of Cosines of interior angles
Sum of square altitudes to N-periodic tangents is invariant
The two types of self-intersected 7-periodics in the Elliptic Billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 534
1 Metric properties of confocal conics
A family of confocal central conics
x2
a2 + k+y 2
b2 + k= 1
k isin R minusa2minusb2 sends througheach point P outside the axesone ellipse and one orthogonallyintersecting hyperbola
The parameters (ke kh) define theelliptic coordinates of P with
minusa2 lt kh lt minusb2 lt ke
P
F1 F2M
We specify the caustic c (semiaxes ac bc) as k = 0 and the ellipse e with semiaxesae be as k = ke =rArr ke = a
2e minus a2c = b2e minus b2c gt 0
Dec 9 2020 Differential Geometry Seminar TU Wien 634
1 Metric properties of confocal conics
From given (ke kh) follows
x2 =(a2 + ke)(a
2 + kh)
d2
y 2 = minus(b2 + ke)(b
2 + kh)
d2
Conversely P = (ae cos t be sin t)with tangent vector
te(t) = (minusae sin t be cos t)and tc(t)=(minusac sin t bc cos t) yield
P
F1 F2M
kh(t) =minus (a2c sin2 t + b2c cos2 t) = minustc(t)2 = minuste(t)2 + ke
Points on confocal ellipses e and c with the same parameter t have the same coordinatekh ie they belong to the same confocal hyperbola
Dec 9 2020 Differential Geometry Seminar TU Wien 734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12Given P = (ae cos t be sin t)the tangents from P to c (k = 0)include the angle θ(t) where
sin2θ(t)
2=
kete(t)2
tanθ(t)
2= plusmn
radicke
tc(t)
cos θ = 1minus 2kete(t)2
=kh(t) + kekh(t)minus ke
Dec 9 2020 Differential Geometry Seminar TU Wien 834
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
For centuries billiards attracted the attention of mathe-maticians beginning with J-V Poncelet and A Cayley
S Tabachnikov Geometry and BilliardsAmerican Mathematical Society 2005
Recently Dan Reznik revitalized the interest bycomputer animations showing the variation of periodicbilliards He identified 40 invariants eg a constantsum of Cosines of interior angles
Sum of square altitudes to N-periodic tangents is invariant
The two types of self-intersected 7-periodics in the Elliptic Billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 534
1 Metric properties of confocal conics
A family of confocal central conics
x2
a2 + k+y 2
b2 + k= 1
k isin R minusa2minusb2 sends througheach point P outside the axesone ellipse and one orthogonallyintersecting hyperbola
The parameters (ke kh) define theelliptic coordinates of P with
minusa2 lt kh lt minusb2 lt ke
P
F1 F2M
We specify the caustic c (semiaxes ac bc) as k = 0 and the ellipse e with semiaxesae be as k = ke =rArr ke = a
2e minus a2c = b2e minus b2c gt 0
Dec 9 2020 Differential Geometry Seminar TU Wien 634
1 Metric properties of confocal conics
From given (ke kh) follows
x2 =(a2 + ke)(a
2 + kh)
d2
y 2 = minus(b2 + ke)(b
2 + kh)
d2
Conversely P = (ae cos t be sin t)with tangent vector
te(t) = (minusae sin t be cos t)and tc(t)=(minusac sin t bc cos t) yield
P
F1 F2M
kh(t) =minus (a2c sin2 t + b2c cos2 t) = minustc(t)2 = minuste(t)2 + ke
Points on confocal ellipses e and c with the same parameter t have the same coordinatekh ie they belong to the same confocal hyperbola
Dec 9 2020 Differential Geometry Seminar TU Wien 734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12Given P = (ae cos t be sin t)the tangents from P to c (k = 0)include the angle θ(t) where
sin2θ(t)
2=
kete(t)2
tanθ(t)
2= plusmn
radicke
tc(t)
cos θ = 1minus 2kete(t)2
=kh(t) + kekh(t)minus ke
Dec 9 2020 Differential Geometry Seminar TU Wien 834
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
A family of confocal central conics
x2
a2 + k+y 2
b2 + k= 1
k isin R minusa2minusb2 sends througheach point P outside the axesone ellipse and one orthogonallyintersecting hyperbola
The parameters (ke kh) define theelliptic coordinates of P with
minusa2 lt kh lt minusb2 lt ke
P
F1 F2M
We specify the caustic c (semiaxes ac bc) as k = 0 and the ellipse e with semiaxesae be as k = ke =rArr ke = a
2e minus a2c = b2e minus b2c gt 0
Dec 9 2020 Differential Geometry Seminar TU Wien 634
1 Metric properties of confocal conics
From given (ke kh) follows
x2 =(a2 + ke)(a
2 + kh)
d2
y 2 = minus(b2 + ke)(b
2 + kh)
d2
Conversely P = (ae cos t be sin t)with tangent vector
te(t) = (minusae sin t be cos t)and tc(t)=(minusac sin t bc cos t) yield
P
F1 F2M
kh(t) =minus (a2c sin2 t + b2c cos2 t) = minustc(t)2 = minuste(t)2 + ke
Points on confocal ellipses e and c with the same parameter t have the same coordinatekh ie they belong to the same confocal hyperbola
Dec 9 2020 Differential Geometry Seminar TU Wien 734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12Given P = (ae cos t be sin t)the tangents from P to c (k = 0)include the angle θ(t) where
sin2θ(t)
2=
kete(t)2
tanθ(t)
2= plusmn
radicke
tc(t)
cos θ = 1minus 2kete(t)2
=kh(t) + kekh(t)minus ke
Dec 9 2020 Differential Geometry Seminar TU Wien 834
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
From given (ke kh) follows
x2 =(a2 + ke)(a
2 + kh)
d2
y 2 = minus(b2 + ke)(b
2 + kh)
d2
Conversely P = (ae cos t be sin t)with tangent vector
te(t) = (minusae sin t be cos t)and tc(t)=(minusac sin t bc cos t) yield
P
F1 F2M
kh(t) =minus (a2c sin2 t + b2c cos2 t) = minustc(t)2 = minuste(t)2 + ke
Points on confocal ellipses e and c with the same parameter t have the same coordinatekh ie they belong to the same confocal hyperbola
Dec 9 2020 Differential Geometry Seminar TU Wien 734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12Given P = (ae cos t be sin t)the tangents from P to c (k = 0)include the angle θ(t) where
sin2θ(t)
2=
kete(t)2
tanθ(t)
2= plusmn
radicke
tc(t)
cos θ = 1minus 2kete(t)2
=kh(t) + kekh(t)minus ke
Dec 9 2020 Differential Geometry Seminar TU Wien 834
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12Given P = (ae cos t be sin t)the tangents from P to c (k = 0)include the angle θ(t) where
sin2θ(t)
2=
kete(t)2
tanθ(t)
2= plusmn
radicke
tc(t)
cos θ = 1minus 2kete(t)2
=kh(t) + kekh(t)minus ke
Dec 9 2020 Differential Geometry Seminar TU Wien 834
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12The signed distances of the sidetQ = P1P2 from the center Oand the pole R1 wrt e have theconstant product minuske For Q1 = (ac cos t
prime1 bc sin t
prime1)
OtQ =minusacbctc(t prime1)
RtQ = R1Q1 =ketc(t prime1)acbc
Dec 9 2020 Differential Geometry Seminar TU Wien 934
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
P1tP1
P2P5
Q1
Q4
Q5
R1
R4
R5
OF1
F2
e
c cprimer2
l1r1
l5
θ12 The side P1P2 (parameters t1 t2)contacts the caustic c iff
sin2t1 minus t22=keaebe
∥∥∥ te(t1 + t22
)∥∥∥2
If t prime1 is the parameter of thetangency point Q1 isin c then
tan t prime1 =bcaeacbe
tant1 + t22
Half angle substitution yields
Dec 9 2020 Differential Geometry Seminar TU Wien 1034
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
1 Metric properties of confocal conics
The half-angle substitution τi = tanti2
yields for P1 and P2 a symmetric biquadraticequation in projective coordinates on e namely
b2ekeτ21τ22 minus b2ca2e(τ21 + τ22 ) + 2(a2eke + a2cb2e)τ1τ2 + b2eke = 0
which defines a 2-2-correspondence between consecute points P1 P2 of a billiard Thesame holds after N iteration between P1 and PN+1
We recall a classical algebraic argument for the Poncelet porism
A 2-2-correspondence (6= id) has at most four fixed points However four fixed pointsare already known as contact points between e and the common (isotropic) tangentswith the caustic c Hence if one N-sided billiard closes then the correspondence isthe identity and each billiard inscribed in e with caustic c must close
Dec 9 2020 Differential Geometry Seminar TU Wien 1134
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
2 Confocal conics and billiards
P1
P2
P3
Q1Q2
O
θi2
θi2
ce
n1
n2
u1u2
ui = 1
S Tabachnikov the key result for the integrability of billiards is the Joachimsthal
integral Je = minus〈ui ni〉 with ui as unit vector of PiPi+1 and ni = (cos tae sin tbe)as a normal vector of e This holds in all dimensions (ni = Api = p
lowasti ) In the plane
Je = minus〈ui ni〉 = minus cos(π2+θi2
)ni = sin
θi2ni =
radickete
teaebe
=
radickeaebe
Dec 9 2020 Differential Geometry Seminar TU Wien 1234
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
2 Confocal conics and billiards
c
The extended sides of a billiard intersect atpoints of confocal ellipses and hyperbolasand form a Poncelet grid
affinely transformed 72-sided periodic billiard with
associated Poncelet grid (G Glaeser B Odehnal
HS The Universe of Conics 7 KB)
Dec 9 2020 Differential Geometry Seminar TU Wien 1334
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
c
c1 t1
t2
t3
t4 Theorem
Given a quadrilateral t1 t4 oftangents to c from A1 B1 isin c1Then the range Rc spanned byc and c1 contains conics c2c3 passing through the remain-ing pairs of opposite vertices(A2 B2) and (A3 B3)
(range = lsquodual pencilrsquo)
= summary of results from M Chasles (1843) W Boumlhm (1961) Izmestiev amp Tabachnikov (2016)
Akopyan amp Bobenko (2017)
Dec 9 2020 Differential Geometry Seminar TU Wien 1434
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
2 Confocal conics and billiards
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
t1
t2
t3
t4
Rt = conics tangent to t1 t4
Rc capRt = c =rArr theyspan a net N (2-parameter set)
In N the line elements of c1 atA1 and B1 span a range whichcontains the rank-1 conic R
R
c
A1B1
c1
c2
c3
A3B3
A2B2
Rc Rt
Dec 9 2020 Differential Geometry Seminar TU Wien 1534
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
2 Confocal conics and billiardsreplacements
B1
A1
A2
B2
A3
B3
R
c
c1
c2
c3
dt1
t2
t3
t4In N the line pencils Ai Bi
and the pencil R (2-fold) spana range which intersects Rc atci The range contains conicssharing the line elements at Aiand Bi
The tangents to ci at Ai and Bipass through RConfocal c c1 =rArr concyclicquadrilateral
This holds also when B2 isin c(t1 = t2)
Dec 9 2020 Differential Geometry Seminar TU Wien 1634
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
2 Confocal conics and billiards
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)2 = [P1 P2] cap [P3 P4] on the confocal hyperbola through Q2
S(2)2 = [P0 P1] cap [P3 P4] isin e(2) on the confocal hyperbola through P2
Dec 9 2020 Differential Geometry Seminar TU Wien 1734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
P1
P2
P3P4
P0
Q1
Q2
Q3
S(1)1
S(1)2
S(2)1
S(2)2
c
e e(1)
e(2)
S(1)i = [Piminus1 Pi ] cap [Pi+1 Pi+2] on the confocal ellipse e(1) for all i
and e(1) is invariant of the initial data ke|1 = ke
(2acbcaebea2cb
2c minus k2e
)2
Dec 9 2020 Differential Geometry Seminar TU Wien 1834
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
2 Confocal conics and billiards
P2
P3
Q2
Q3
Q1
S(1)2
S(1)3
S(2)3
ww
w
w1
w1
ce
e(1)
e(2)
r2
l2
r3r3
l3
l3Surprisingly the distance w
between S(1)2 and the contact
point with the incircle isinvariant the same for w1
Gravesrsquos construction =rArrDe = Qiminus1Pi + PiQi minus
Qiminus1Qi
= ri + li minusQiminus1Qi
is constantSimilarly for e(1) follows
De|1 =Q1S(1)2 + S
(1)2 Q3 minus
Q1Q3 = (r2+l2+w) + (r3+l3+w)minus
Q1Q3= 2De + 2w
Dec 9 2020 Differential Geometry Seminar TU Wien 1934
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
P2 P3
P4P1
Q2Q4
Q1
e
c
e(1)
e(2)
The same invariants show upon the sphere All circulararcs in black have the samelength The same lsquoin greenrsquo
In the plane w =2aebe
radick3e
a2cb2c minus k2e
Dec 9 2020 Differential Geometry Seminar TU Wien 2034
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
2 Confocal conics and billiards
P2
P3P1
Qprime1
Qprime2
P prime2P prime1
Q2 Q1
R2
R1
O
e
cr3
l2r2
l1
θ22 For each billiard P0P1P2 in the ellipse e with causticc there exists a conjugate
billiard P prime0Pprime1Pprime2
The axial scaling with c rarr eα Qi 7rarr P primei Qprimeiminus1 7rarr Pi transforms tangents P1P2 ofc to tangents P prime1P
prime2 This
results from the symmetrybetween ti and t primei in theequation
bcae cos ti cos tprimei +acbe sin ti sin t
primei = acbc which expresses that Pi lies on the tangent
to c at Qi and P primei on the tangent at Qprimei
Dec 9 2020 Differential Geometry Seminar TU Wien 2134
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
2 Confocal conics and billiards
P prime1
P prime2P2P3
P1
Qprime1Qprime2
Q1
Q2
S(1)1
S(1)2
S(1)3
c
e(1)
e1
0
1234
14
7858
One might say point P primei ishalfway from Pi to Pi+1
In the sequence of para-meters t1 t
prime1 t2 t
prime2
for P1 Pprime1(Q1) P2 P
prime2(Q2)
the transition Pi 7rarr P primeimeans a shift ti 7rarr t primei
I Izmestiev S Tabachnikov (2017) There exists a canonical parametrization ofe such that the billiard transformation Pi rarr Pi+1 corresponds to a shiftui rarr ui+1 = ui + 2∆u
Above an example of canonical parameters P1 sim 0 P2 sim 12 P3 sim 1
Dec 9 2020 Differential Geometry Seminar TU Wien 2234
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
ce
e(1)
e(2)
The turning number τ of aperiodic billiard counts the loopsaround the center
Theorem
(i) A periodic billiard with even Nand odd τ is centrally symmetric
(ii) For odd N and odd τ thebilliard is centrally symmetric tothe conjugate billiard
(iii) For odd N and even τthe billiard conincides with theconjugate billiard
Dec 9 2020 Differential Geometry Seminar TU Wien 2334
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
3 Periodic N-sided billiards
P1
P2
P3
P7
P8
P9
Q1
Q2
Q6Q7
Q8
Q9
S(1)1
S(1)2
S(1)7
S(1)8
S(1)9
S(2)1
S(2)9
S(2)8
l2
r7
ce
e(1)
e(2)
From Gravesrsquo theorem
De = Qiminus1Pi + PiQi minusQiminus1Qi
follows for the perimeter of the N-sided billiard
Le = N middotDe + τ middot Pcwith Pc as perimeter of e
Ivoryrsquos theorem implies for odd
N = 2n + 1 the length li equalssymmetric l primei+n of the conjugatebilliard and ri+n+1 of the originalone
Theoremsumli =
sumri = Le2
Dec 9 2020 Differential Geometry Seminar TU Wien 2434
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
3 Periodic N-sided billiards
Piminus1
Pi
Pi+1
Qiminus1Qi
Fiminus1Fi
O
θi2
θi2
ce
PiFi + PiFiminus1 =2aebete(ti)2
radicke
Le =sumNi=1
(PiFi + PiFiminus1
)=rArr
Le =aeberadicke
Nsum
i=1
2kete(ti)2
=aeberadicke
Nsum
i=1
(1minus cos θi)
Theorem [Akopyan Schwartz Tabachnikov Bialy]sumNi=1 cos θi = N minus
radickeaebe
Le
WithsumNi=1
1
te(ti)2also
sumNi=1OtP
2and
sumNi=1 κe(ti)
23 are invariant
Dec 9 2020 Differential Geometry Seminar TU Wien 2534
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
4 A switch to Analysis
F2F1
P2 tP
Q1Q2
r2
l2
e
c
vt1vn1
vt2vn2
v2
ω1
ω2
θ22
With a billiard motion we denotea variation of the billiard and thePoncelet grid induced by the variationof a single vertex
According to Gravesrsquo constructionthe v2 of P2 can be decomposed as
v2 = vt1 + vn1 = vt2 + vn2
where due to the constant length
vt2 = vt1 and vn2 = vn1l2ω2 = r2ω1 hence
ω1ω2=l2r2
If the billiard is periodic then the product of all ratios liri yields
l1r1middot l2r2middot middot middot lNrN=ωNω1middot ω1ω2middot middot middot ωNminus1ωN
= 1 hence l1l2 lN = r1r2 rN
Dec 9 2020 Differential Geometry Seminar TU Wien 2634
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
For periodic billiards a givenvelocity vector v2 of anyvertex defines all velocities
In terms of the exterior anglesθ1 θN we obtain
sinθ22=l2 ω2v2=r2ω1v2
and
cosθ22=vt|2v2
where
v2 = v2 vt|2 = vt|2From P1P2 perp Q1R1 follows
R1Q1 = l1 tanθ12= r2 tan
θ22
Dec 9 2020 Differential Geometry Seminar TU Wien 2734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
4 A switch to Analysis
P1
P2
P3
P4
P5
Q1Q2
Q3
Q4
Q5
R1
R2
R3
R4
R5
e
c
l1
r2l2
r3
θ12
θ12
θ22
θ22
ω1
ω2
ω3ω4
ω5
v2
vt2vn|2
v1
v3
v4
v5
After some manipulationsfollows
vt|1 tan2 θ12= vt|2 tan
2 θ22
middot middot middot = vt|i ketc(ti)2
= C
Instead of a free choice of v2we set C = ke
vt|i=tc2 vi=tc tevn|i = vi sin
θ
2= tc
radicke
for all t = ti and for all confocalellipses e
Dec 9 2020 Differential Geometry Seminar TU Wien 2834
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
4 A switch to Analysis
c
e
To each point
p = (x y) = (ae cos t be sin t)
we assign a velocity vector
v = tc te =radica2c sin
2 t + b2c cos2 t
(minusaeybebex
ae
)
Theorem
This vector field defines an infinitesimal motion which preserves confocal ellipses andpermutes the confocal hyperbolas and the tangents of the caustic c
The infinitesimal motion generates a one-parameter Liegroup Γ which carries out thebilliard transformation along e and simultaneously that of the associated Poncelet grid
Dec 9 2020 Differential Geometry Seminar TU Wien 2934
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
4 A switch to Analysis
c
e
Q
tQ
vn We set v =d p
du= p Then u is a
canonical parameter of e and of Γie γ(u2) γ(u1) = γ(u1 + u2)v(t)=tc(t) te(t) = t te(t)
t =dt
du=
radica2c sin
2 t + b2c cos2 t
Proof Γ permutes hyperbolas since t (and kh) is independent of e (and ke)
The condition bcae cos t cos tprime + acbe sin t sin t prime = acbc is equivalent to the fact that
P = (ae cos t be sin t) lies on the tangent tQ of Q = (ac cos tprime + bc sin t prime) Differen-
tiation by u yields an identity Hence Γ permutes the tangents of c
Dec 9 2020 Differential Geometry Seminar TU Wien 3034
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
4 A switch to Analysis
In order to express the action of γ(u) isin Γ on (ae cos t be sin t) we integrate
dt
du=
radica2c sin
2 t + b2c cos2 t =
radica2c sin
2 t + (a2c minus d2) cos2 t = acradic1minusm2 cos2 t
withm = dac lt 1 as numeric eccentricity of the caustic c We substitute ϕ = tminusπ2
and get under the initial condition ϕ = 0 for u = 0
dϕradic1minusm2 sin2ϕ
= ac du hence ac u(ϕ) = F (ϕm) =
int ϕ
0
dϕradic1minusm2 sin2ϕ
with F (ϕm) as the elliptic integral of the first kind with the modulusm This functionshows the canonical coordinate u in terms of ϕ with the quarter period
K = ac u(π
2
)=
int π2
0
dϕradic1minusm2 sin2ϕ
Dec 9 2020 Differential Geometry Seminar TU Wien 3134
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
4 A switch to Analysis
For the sake of simplicity we define u(ϕ) = ac u(ϕ) as a new canonical coordinate
The inverse function of u = F (ϕm) the Jacobian amplitude ϕ = am(u ) leads tothe Jacobian elliptic functions
sn u = sin(am(u )) and cn u = cos(am(u ))
which can be extended in R to periodic functions with period 4K
Dec 9 2020 Differential Geometry Seminar TU Wien 3234
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
4 A switch to Analysis
Theorem
For the ellipse c with semiaxes (ac bc) and eccentricity d =radica2c minus b2c and all confocal
ellipses e with semiaxes (ae be) the inscribed billiards with caustic c can be canonicallyparametrized as (minusae sn u be cn u ) using the Jacobian elliptic functions to themodulus m = dac
If bc = be cn(∆u ) then the vertices of the billiard in e have the canonical parametersu = (u0 + 2k∆u ) for k isin Z and any given initial u0
Conversely we obtain an N-sided billiard withturning number τ where gcd(N τ) = 1 bythe choice ∆u =
2τK
Nwith K as the complete
elliptic integral of the first kind to the modulusm provided that be = bccn(∆u )
Q1(∆u )
P1(0)
P1(2∆u )
y
c
e
Dec 9 2020 Differential Geometry Seminar TU Wien 3334
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
Schoumlnbrunn Castle Vienna
Thank you for your attention
Dec 9 2020 Differential Geometry Seminar TU Wien 3434
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
References
References
[1] AW Akopyan AI Bobenko Incircular nets and confocal conics Trans AmerMath Soc Nov 16 2017 (arXiv160204637v2[mathDS]27Oct2017)
[2] A Akopyan R Schwartz S Tabachnikov Billiards in ellipses revisitedarXiv200102934v2[mathMG] 2020
[3] M Bialy S Tabachnikov Dan Reznikrsquos identities and morearXiv200108469v2[mathDG] 2020
[4] W Boumlhm Ein Analogon zum Satz von Ivory Ann Mat Pura Appl (4) 54221ndash225 (1961)
Dec 9 2020 Differential Geometry Seminar TU Wien 3534
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
[5] W Boumlhm Verwandte Saumltze uumlber Kreisvierseitnetze Arch Math 21 326ndash330(1970)
[6] M Chasles Proprieacuteteacutes geacuteneacuterales des arcs drsquoune section conique dont la differenceest rectifiable Comptes Rendus hebdomadaires de seacuteances de lrsquoAcadeacutemie dessciences 17 838ndash844 (1843)
[7] M Chasles Reacutesumeacute drsquoune theacuteorie des coniques spheacuteriques homofocalesComptes Rendus des seacuteances de lrsquoAcadeacutemie des Sciences 50 623ndash633 (1860)
[8] A Chavez-Caliz More about areas and centers of Poncelet polygonsarXiv200405404 (2020)
[9] G Glaeser H Stachel B Odehnal The Universe of Conics Springer SpectrumBerlin Heidelberg 2016
[10] Ph Griffiths J Harris On Cayleyrsquos explicit solution to Ponceletrsquos porismLrsquoEnseignement Matheacutematique 241-2 31ndash40 (1978)
Dec 9 2020 Differential Geometry Seminar TU Wien 3634
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734
[11] L Halbeisen N Hungerbuumlhler A Simple Proof of Ponceletrsquos Theorem (on theoccasion of its bicentennial) Amer Math Monthly 1226 537ndash551 (2015)
[12] I Izmestiev S Tabachnikov Ivoryrsquos Theorem revisited J Integrable Syst 21xyx006 (2017) httpsdoiorg101093integrxyx006
[13] D Reznik R Garcia J Koiller Forty New Invariants of N-Periodics in the EllipticBilliard arXiv200412497[mathDS] 2020
[14] H Stachel Recalling Ivoryrsquos Theorem FME Transactions 47 No 2 355ndash359(2019)
[15] S Tabachnikov Geometry and Billiards American Mathematical SocietyProvidenceRhode Island 2005
Dec 9 2020 Differential Geometry Seminar TU Wien 3734