Post on 27-Jul-2020
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September the 22nd, 2016, Amsterdam, The Netherlands
On the Collatz conjecture: a contracting Markov walk
on a directed graph.
Timoteo Carletti & Duccio Fanelli
www.unamur.be timoteo.carletti@unamur.be
Acknowledgements
Several colleagues, among which:
Jean-Charle Delvenne, Craig Alan Feinstein, Steffen Kionke, Vassilis Papanicolaou and Cédric Villani
IAP$VII/19$)$DYSCO$
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The Collatz map T (Lothar Collatz, 1937)
8n 2 N T (n) =
(n2 if n is even
3n+ 1 if n is odd.
1 ���!even
2/2 = 1��!odd
3⇥ 1 + 1 = 4���!even
4/2 = 2
��!odd
3⇥ 3 + 1 = 103 ���!even
10/2 = 5��!odd
5⇥ 3 + 1 = 16
16 ���!even
16/2 = 8 ���!even
8/2 = 4 ! 2 ! 1
! 22! 11! 34! 17! 52! 267 ! 13! 40! 20
! 10! 5! 16! 8 ! 4! 2! 1
Numerically verified up to 1.9x1017 T. Oliveira E Silva, Math. Comp. 68 No. 1 (1999), 371
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The Collatz map T (Lothar Collatz, 1937)
Conjecture For any given positive integer the iterates of the Collatz map eventually converge to the period-3 cycle {1,2,4} as .
n0 2 Nnk = T �k(n)
k ! 1
J.C. Lagarias, "The Ultimate Challenge: the 3x+1 problem". American Mathematical Society, (2010).
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Main ideas of the proof
1) Introduce the third iterate of the Collatz map and consider the equivalence classes of integer numbers modulo 8;
2) Define a finite state Markov chain with a suitable invariant probability measure, whose transition probabilities reflect the deterministic map;
3) Prove a weak version of the conjecture: diverging orbits cannot exist due to the bound on the average stationary distribution;
4) Prove the conjecture by excluding other periodic orbits.
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1) The third iterate of the Collatz map S=To3
B(k, 8) := {n 2 N : 9m 2 N, n = 8m+ k} k 2 {0, . . . , 7}
The 3-cycle {1,2,4} becomes a fixed point
S(n) =
8>>>>>>>>>>>>><
>>>>>>>>>>>>>:
n8 if n 2 B(0, 8)6n+2
8 if n 2 B(1, 8)6n+4
8 if n 2 B(2, 8)36n+20
8 if n 2 B(3, 8)6n+8
8 if n 2 B(4, 8)6n+2
8 if n 2 B(5, 8)6n+4
8 if n 2 B(6, 8)36n+20
8 if n 2 B(7, 8) .
8n 2 N
contraction by 1/8contraction by 3/4contraction by 3/4
contraction by 3/4contraction by 3/4contraction by 3/4
expansion by 9/2
expansion by 9/2
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2) A finite states Markov process
8 symbols alphabet: B(k, 8) k = 0, . . . , 7
Transition probabilities:q
⇤ij = P [S(x) 2 B(j, 8)|x 2 B(i, 8)]
q⇤ij :=µinv
⇥B(i, 8) \ S�1B(j, 8)
⇤
µinv [B(i, 8)]
invariant (probability) measureµinv
Transition prob of Sok is the kth power of the transition prob of S
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2) Markov process. Transition probabilities: an intuition
Let thenn = 8k 2 B(0, 8) S(n) = n/8 = k
with probability 1/8, S(n) belongs to any one of the eight classes
with probability 1/2, S(n) belongs to
Let thenn = 3 + 8k 2 B(3, 8)S(n) = (36n+ 20)/8 = 16 + 36k = 8(4k + 2) + 4k
B(0, 8) B(4, 8)or to
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2) Markov process. Invariant measure
Let , there exist and n 2 N s0, . . . , sk�1 2 {0, . . . , 7}k � 1
n = sk�18k�1 + · · ·+ s18 + s0
⌫(s0) =1
6if s0 = 0, 2, 4, 6 and ⌫(s0) =
1
12if s0 = 1, 3, 5, 7.
µinv(sk�1 . . . s1s0) =1
2k1
7
1
8k�2⌫(s0)
Note: on all integers.µinv > 0
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2) Markov process. Transition probabilities
7
0
6
5
4
3
2
1
Q⇤ =
0
BBBBBBBBBB@
18
18
18
18
18
18
18
18
0 14 0 1
4 0 14 0 1
414 0 1
4 0 14 0 1
4 012 0 0 0 1
2 0 0 014 0 1
4 0 14 0 1
4 014 0 1
4 0 14 0 1
4 00 1
4 0 14 0 1
4 0 14
0 0 12 0 0 0 1
2 0
1
CCCCCCCCCCA
Q* is a stochastic matrix
8i = 0, . . . , 7X
j
q⇤ij = 1
q⇤ij :=µinv
⇥B(i, 8) \ S�1B(j, 8)
⇤
µinv [B(i, 8)]
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2) Markov process. Some properties
The Markov process with stochastic matrix Q* is ergodic.
visiting frequencies of classes B(k, 8)Contracting/expanding property of S
and
fQ⇤ =
✓1
8
◆1/6 ✓3
4
◆2/3 ✓9
2
◆1/6
=3
4< 1
average contracting/expanding factor:
~PS = (1/6, 1/12, 1/6, 1/12, 1/6, 1/12, 1/6, 1/12)
The left eigenvector associated to is � = 1 ~PSQ⇤ = ~PS
µinv(B(k, 8)) = PS,k
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3) Weak Collatz conjecture: all orbits are bounded
ProofAssume there exists a diverging orbit for S. Observe it is a possible realisation of the Markov chain.
Given any , then there exists a positive integer M(n0) such that for all k>0.
n0 2 NS�k(n0) M(n0)
limk!1
#{0 j k � 1 : S�j(n0) 2 B(i, 8)}k
= µinv (B(i, 8)) 8i 2 {0, . . . , 7}
Because of the ergodic theorem, for - almost allµinv n0 2 N
Contradiction with the unboundedness of the orbit.
Hence for this orbits it holds true: fQ⇤ < 1
µinvn0 2 N
The existence of zero measure sets can be ruled out because for allµinv(n0) > 0
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4) Collatz cycle: {1,2,4} is the unique attracting cycle
Proof
Assume there exists a L-periodic orbit for S with L > 1. Observe it is a possible realisation of the Markov chain, hence it should infinitely visit the 8 classes.
No periodic orbits of period different from 1 exist for S.
Invoking the ergodic theorem this orbit should have fQ⇤ < 1
Contradicting the periodicity (that would require )fQ⇤ = 1
µinvThe existence of zero measure sets can be ruled out as previously done
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4’) Markov process with absorbing state
Let us observe that once we have proved the weak Collatz conjecture we can conclude using the following finite state Markov process
8
7
0
6
5
4
3
2
1
8>>>>>>>>>>>>>>><
>>>>>>>>>>>>>>>:
C0 = B(0, 8)C1 = B(1, 8) \ {1}C2 = B(2, 8) \ {2}C3 = B(3, 8)C4 = B(4, 8) \ {4}C5 = B(5, 8)C6 = B(6, 8)C7 = B(7, 8)C8 = {1, 2, 4}
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Help save Dmitry!
Vsevolod Salnikov is currently postdoc in the Namur Center for Complex Systems. His son
Dmitry has been diagnosed with acute lymphoblastic leukaemia . The best chances of beating the disease are given by a cutting edge CAR T-cell therapy, but the treatment is currently only accessible in Seattle (USA). The family is
currently trying to collect the sum for the treatment, as it is not covered by standard
insurances nor the Belgian health system. Any donation is very, very welcome!
More information on : http://dmitry.be/ or ask the Namur people (Timoteo Carletti, Sarah de Nigris, Renaud Lambiotte, Michael
Schaub, etc.)
www.unamur.be timoteo.carletti@unamur.be
September the 19th, 2016, Amsterdam, The Netherlands
On the Collatz conjecture: a contracting Markov walk on a directed
graph.
Timoteo Carletti & Duccio Fanelli
www.unamur.be timoteo.carletti@unamur.be
Markov process. Invariant measure
µinv(B(�, 8)) =1
6if � = 0, 2, 4, 6,
µinv(B(�, 8)) =1
12if � = 1, 3, 5, 7 and µinv(N) = 1
µinv
⇥S�1B(j, 8m)
⇤= µinv [B(j, 8m)]
For any and m � 1 j = 0, . . . , 8m � 1
Probability measure
Invariance