Proof Mining and the `Lion-Man' Game · 2020. 12. 3. · Proof Mining and the ‘Lion-Man’ Game...

Proof Mining and the ‘Lion-Man’ Game

Ulrich Kohlenbach(joint work with Genaro Lopéz-Acedo and Adriana Nicolae)1

Department of Mathematics

Proof Theory Virtual Seminar, 2.12.2020

1To appear in: Pacific Journal of Mathematics; arXiv:1806.04496U. Kohlenbach (joint work with G. Lopéz-Acedo and A. Nicolae) Proof Mining and the ‘Lion-Man’ Game

Pursuit-Evasion Games: The Lion and Man Game

The lion and man problem, going back to R. Rado, is one of the mostchallenging pursuit-evasion games. Littlewood’s Miscellany it isdescribed as follows:

A lion and a man in a closed circular arena have equalmaximum speeds. What tactics should the lion employ to besure of his meal?

Very similar problems have appeared under different names in theliterature (e.g. the robot and the rabbit or the cop and the robber).

The analysis of the game is closely tied to the geometric structure of thedomain where the game is played.

This fact, as well as the potential applications in different fields such asrobotics, biology and random processes.

U. Kohlenbach (joint work with G. Lopéz-Acedo and A. Nicolae) Proof Mining and the ‘Lion-Man’ Game

Many variants of the game:

continuous and discrete,

one or more evaders hunted by one or more pursuers,

physical capture or ε-capture,

different degrees of freedom in the movement of the lion.

We focus on a discrete-time equal-speed game and ε-capture.

The domain X of our game for now is a geodesic space X . Initially, thelion and the man are located at L0,M0 ∈ X .

Fix upper bound D > 0 on the distance the lion and the man may jump.

After n steps, the lion moves from Ln to Ln+1 along a geodesic from Ln

to Mn, i.e. d(Ln,Mn) = d(Ln, Ln+1) + d(Ln+1,Mn), s.t. its distanceto Ln equals minD, d(Ln,Mn).

The man moves from Mn to any point Mn+1 ∈ X which is withindistance D.

Given a metric space X , we say that the lion wins iflimn→∞ d(Ln+1,Mn) = 0 for any pair of sequences (Ln), (Mn)

satisfying the previous metric conditions for any D > 0. Otherwise theman wins.

The point of departure of our research

Let (X , d) be a uniquely geodesic space. Then the move of the lion isuniquely determined

Ln+1 := (1− λn)Ln + λnMn, λn := minD, d(Ln,Mn)/d(Ln,Mn).

Lopéz-Acedo/Nicolae/Piątek (Geom. Dedicata 2019):if X is a compact uniquely geodesic space with the betweennessproperty, then the lion wins i.e. lim

n→∞d(Ln+1,Mn) = 0.

The proof proceeds by an induction along an iterated use of sequentialcompactness i.e. of arithmetical comprehension!

n→∞d(Ln+1,Mn) = 0.

Proof-Mining

Based on general logical metatheorems: one can extract anexplicit rate of convergence if one upgrades ‘uniquely geodesic’and ‘betweenness property’ to ‘uniform uniquely geodesic (withmodulus)’ and ‘uniform betweenness property (with modulus)’.

With these upgrades the assumption of compactness can bereplaced by boundedness!

Even the uniqueness of geodesics can be dropped.

Proof mining provides an explicit rate of convergence which onlydepends on a given modulus of uniform betweenness Θ (inaddition to b ≥ diam(A), ε > 0,D).

Crucial: lim d(Ln+1,Mn) = 0 ∈ Π02 since the sequence is

nonincreasing.

Proof-Mining

nonincreasing.

Proof-Mining

nonincreasing.

Proof-Mining

nonincreasing.

Proof-Mining

nonincreasing.

Proof mining, moreover, guarantees the construction of Θ for

uniformly convex geodesic spaces (with convex metric)depending only on a modulus of uniform convexity (extracted fromproof of betweenness for strictly normed spaces and geodesic spaceswith convex metric due to A. Nicolae).

Ptolemy spaces (extracted from proof of betweenness due to A.Nicolae)

A particular nonstrictly normed space (R3, ‖ · ‖DW) (extractedfrom proof of betweenness due to Diminnie and White).

Basics of Proof Mining

Shift of emphasis (G. Kreisel ≥ 1951): use proof theory not forfoundational purposes but to extract new information from proofs ofexistential statements.

‘What more do we know if we have proved a theorem by restricted meansthan if we merely know that it is true?’ (G. Kreisel)

Goals e.g. for conclusions ∀x∃y A(x , y):

effective bounds,

algorithms,

continuous dependency or full independence from certain parameters,

generalizations of proofs: weakening of premises.

effective bounds,

algorithms,

effective bounds,

algorithms,

effective bounds,

algorithms,

effective bounds,

algorithms,

effective bounds,

algorithms,

Formal systems for analysis with abstract spaces X

Types: (i) N,X are types, (ii) with ρ, τ also ρ→ τ is a type.

PAω,X is the extension of Peano Arithmetic to all types.

Aω,X :=PAω,X+DC, where

DC: axiom of dependent choice for all types

Implies full comprehension for numbers (higher order arithmetic).

Aω[X , d , . . .] results by adding constants dX , . . . with axioms expressingthat (X , d , . . .) is a nonempty metric, hyperbolic . . . space.

Majorization

y , x functionals of types ρ, ρ := ρ[N/X ] and aX of type X :

xN &aN yN :≡ x ≥ y

xN &aX yX :≡ x ≥ d(y , a).

For complex types ρ→ τ this is extended in a hereditary fashion.

Example:

f∗ &aX→X f ≡ ∀n ∈ N, x ∈ X[n ≥ d(a, x)→ f∗(n) ≥ d(a, f(x))].

f : X → X is nonexpansive (n.e.) if d(f (x), f (y)) ≤ d(x, y).

Then λn.n + b &aX→X f , if d(a, f (a)) ≤ b.

Normed linear case: a := 0X .

Majorization

xN &aX yX :≡ x ≥ d(y , a).

Example:

Majorization

xN &aX yX :≡ x ≥ d(y , a).

Example:

Majorization

xN &aX yX :≡ x ≥ d(y , a).

Example:

Majorization

xN &aX yX :≡ x ≥ d(y , a).

Example:

Goal: Effective bounds for∀x ∈ P,K ,X ,XX ,XN . . . ∃n ∈ NA(x , n)-theorems.

Restriction: Because of classical logic: in general A existential.

If A is existential, then general logical metatheorems (K. 2005)guarantee the extractability of effective bounds on ‘∃’ that areindependent from parameters x from

compact metric spaces K (if separability is used) and

metrically bounded subsets of abstract spaces X that are notassumed to be separable (provided X belongs to a sufficientlyuniformly axiomatizable class of spaces).

Examples of such spaces X : metric, geodesic, normed, Hilbert,uniformly convex uniformly smooth, hyperbolic, CAT(0), Ptolemy spaces,abstract Lp and C (K ) spaces ... (not: separable, strictly convex orsmooth spaces).

Also several metric structures X1, . . . ,Xn simultaneously (Günzel/K.).

Also several metric structures X1, . . . ,Xn simultaneously (Günzel/K.).U. Kohlenbach (joint work with G. Lopéz-Acedo and A. Nicolae) Proof Mining and the ‘Lion-Man’ Game

Small types (over N,X ): N, N→ N, X , N→ X , X → X .

Theorem (K., Trans.AMS 2005, Gerhardy/K.,Trans.AMS 2008)

Let P,K be Polish resp. compact metric spaces, A∃ ∃-formula,τ small. If Aω[X , d ] proves

∀x ∈ P∀y ∈ K∀zτ∃vNA∃(x, y, z, v),

then one can extract a computable ϕ : NN × N(N) → N s.t. thefollowing holds in every nonempty metric space: for all representativesrx ∈ NN of x ∈ P and all zτ and z∗ ∈ N(N) s.t. ∃a ∈ X (z∗ &a

τ z):

∀y ∈ K∃v ≤ ϕ(rx, z∗)A∃(x, y, z, v).

In the normed case: a := 0X .

Small types (over N,X ): N, N→ N, X , N→ X , X → X .

Theorem (K., Trans.AMS 2005, Gerhardy/K.,Trans.AMS 2008)

Let P,K be Polish resp. compact metric spaces, A∃ ∃-formula,τ small. If Aω[X , d ] proves

∀x ∈ P∀y ∈ K∀zτ∃vNA∃(x, y, z, v),

then one can extract a computable ϕ : NN × N(N) → N s.t. thefollowing holds in every nonempty metric space: for all representativesrx ∈ NN of x ∈ P and all zτ and z∗ ∈ N(N) s.t. ∃a ∈ X (z∗ &a

τ z):

∀y ∈ K∃v ≤ ϕ(rx, z∗)A∃(x, y, z, v).

In the normed case: a := 0X .

Relations to ultrapowers

The rule stated in the metatheorem can (without the computabilityof the bound) also be stated as a ‘nonstandard’ uniformboundedness axiom ∃-UBX .

While ∃-UBX is in general false but can be added to the sourcesystem still resulting in classical correct effective bounds (K. 2008).In the form of a ‘bounded collection principle’ ∃-UBX has recentlybeen used to replace certain weak sequential compactnessarguments (Ferreira, Leuştean, Pinto, Adv. Math. 2019).∃-UBX and ultraproducts: see Günzel, K., Adv. Math. 2016.∃-UBX proves that every bounded metric (geodesic) space whichhas the betweenness property (is uniquely geodesic) has the uniformbetweenness property (is uniformly uniquely geodesic)

Recent Survey:K., Proof-Theoretic Methods in Nonlinear Analysis. Proc. ICM 2018.

The rule stated in the metatheorem can (without the computabilityof the bound) also be stated as a ‘nonstandard’ uniformboundedness axiom ∃-UBX .While ∃-UBX is in general false but can be added to the sourcesystem still resulting in classical correct effective bounds (K. 2008).

In the form of a ‘bounded collection principle’ ∃-UBX has recentlybeen used to replace certain weak sequential compactnessarguments (Ferreira, Leuştean, Pinto, Adv. Math. 2019).∃-UBX and ultraproducts: see Günzel, K., Adv. Math. 2016.∃-UBX proves that every bounded metric (geodesic) space whichhas the betweenness property (is uniquely geodesic) has the uniformbetweenness property (is uniformly uniquely geodesic)

The rule stated in the metatheorem can (without the computabilityof the bound) also be stated as a ‘nonstandard’ uniformboundedness axiom ∃-UBX .While ∃-UBX is in general false but can be added to the sourcesystem still resulting in classical correct effective bounds (K. 2008).In the form of a ‘bounded collection principle’ ∃-UBX has recentlybeen used to replace certain weak sequential compactnessarguments (Ferreira, Leuştean, Pinto, Adv. Math. 2019).

∃-UBX and ultraproducts: see Günzel, K., Adv. Math. 2016.∃-UBX proves that every bounded metric (geodesic) space whichhas the betweenness property (is uniquely geodesic) has the uniformbetweenness property (is uniformly uniquely geodesic)

The rule stated in the metatheorem can (without the computabilityof the bound) also be stated as a ‘nonstandard’ uniformboundedness axiom ∃-UBX .While ∃-UBX is in general false but can be added to the sourcesystem still resulting in classical correct effective bounds (K. 2008).In the form of a ‘bounded collection principle’ ∃-UBX has recentlybeen used to replace certain weak sequential compactnessarguments (Ferreira, Leuştean, Pinto, Adv. Math. 2019).∃-UBX and ultraproducts: see Günzel, K., Adv. Math. 2016.

∃-UBX proves that every bounded metric (geodesic) space whichhas the betweenness property (is uniquely geodesic) has the uniformbetweenness property (is uniformly uniquely geodesic)

The rule stated in the metatheorem can (without the computabilityof the bound) also be stated as a ‘nonstandard’ uniformboundedness axiom ∃-UBX .While ∃-UBX is in general false but can be added to the sourcesystem still resulting in classical correct effective bounds (K. 2008).In the form of a ‘bounded collection principle’ ∃-UBX has recentlybeen used to replace certain weak sequential compactnessarguments (Ferreira, Leuştean, Pinto, Adv. Math. 2019).∃-UBX and ultraproducts: see Günzel, K., Adv. Math. 2016.∃-UBX proves that every bounded metric (geodesic) space whichhas the betweenness property (is uniquely geodesic) has the uniformbetweenness property (is uniformly uniquely geodesic)

Proof Mining applied to the‘Lion-Man’ game

Metric spaces with the betweenness and uniformbetweenness properties

The concept of ‘betweenness’ can be formulated in arbitrary metricspaces:

Definition (Diminnie and White 1981)

Let (X , d) be a metric space. X satisfies the betweenness property if forany distinct points x , y , z ,w ∈ X

d(x , y) + d(y , z) ≤ d(x , z)

d(y , z) + d(z ,w) ≤ d(y ,w)

⇒ d(x , z) + d(z ,w) ≤ d(x ,w).

Logical form (put in prenex normal form):

∀x, y , z,w ∈ X ∀k,m ∈ N ∃n ∈ Nsepx, y , z,w ≥ 2−k∧d(x, y) + d(y , z) ≤ d(x, z) + 2−n∧d(y , z) + d(z,w) ≤ d(y ,w) + 2−n

→d(x, z)+d(z,w)<d(x,w)+2−m

,where

(. . .)is a purely existential formula A∃.

Logic bound extraction theorems extract from (suitable) proofs of Xsatisfying the betweenness property, a bound (and hence realizer) for∃n ∈ N which only depends on k,m and majorants for x, y , z,w .

Logical form (put in prenex normal form):

∀x, y , z,w ∈ X ∀k,m ∈ N ∃n ∈ Nsepx, y , z,w ≥ 2−k∧d(x, y) + d(y , z) ≤ d(x, z) + 2−n∧d(y , z) + d(z,w) ≤ d(y ,w) + 2−n

→d(x, z)+d(z,w)<d(x,w)+2−m

,where

(. . .)is a purely existential formula A∃.

Logic bound extraction theorems extract from (suitable) proofs of Xsatisfying the betweenness property, a bound (and hence realizer) for∃n ∈ N which only depends on k,m and majorants for x, y , z,w .

In metric setting (taking as reference point e.g. x) any b ∈ N s.t.b ≥ diamx , y , z ,w provides such a majorant. This gives rise to thefollowing notion (expressed for convenience in ε/δ-style):

Definition (K., Lopéz-Acedo, Nicolae 2019)

A metric space (X , d) satisfies the uniform betweenness property withmodulus Θ : (0,∞)3 → (0,∞) if

∀ε, a, b > 0∀x, y , z,w ∈ X

sepx, y , z,w ≥ a ∧ diamx, y , z,w ≤ b

d(x, y) + d(y , z) ≤ d(x, z) + Θ(ε, a, b)

d(y , z) + d(z,w) ≤ d(y ,w) + Θ(ε, a, b)

⇒ d(x, z) + d(z,w) ≤ d(x,w) + ε

In metric setting (taking as reference point e.g. x) any b ∈ N s.t.b ≥ diamx , y , z ,w provides such a majorant. This gives rise to thefollowing notion (expressed for convenience in ε/δ-style):

Definition (K., Lopéz-Acedo, Nicolae 2019)

A metric space (X , d) satisfies the uniform betweenness property withmodulus Θ : (0,∞)3 → (0,∞) if

∀ε, a, b > 0 ∀x, y , z,w ∈ X

sepx, y , z,w ≥ a ∧ diamx, y , z,w ≤ b

d(x, y) + d(y , z) ≤ d(x, z) + Θ(ε, a, b)

d(y , z) + d(z,w) ≤ d(y ,w) + Θ(ε, a, b)

⇒ d(x, z) + d(z,w) ≤ d(x,w) + ε

Definition (Lion-Man Game in general metric spaces)

Let X be a metric space, D > 0 and Let (Mn), (Ln) be sequences in Xs.t. for all n ∈ N

d(Mn,Mn+1) ≤ D, d(Ln+1, Ln) + d(Ln+1,Mn) = d(Ln,Mn),

d(Ln, Ln+1) = minD, d(Ln,Mn).

Then 〈(Mn), (Ln)〉 is called a Lion-Man game with speed D > 0.

Let X be a b-bounded metric space with the uniform betweennessproperty with modulus Θ satisfying

Θ(ε) := Θ(ε, ε, b) ≤ ε for all ε > 0.

For D > 0 let N ∈ N be s.t. b + 1 < ND.

Θ(ε) := Θ(ε, ε, b) ≤ ε for all ε > 0.

Theorem (K./Lopéz-Acedo/Nicolae 2019)

Let X be a bounded metric space with the uniform betweenness propertyand 〈(Mn), (Ln)〉 be an arbitrary Lion-Man game with speed D > 0.Then the Lion approaches the man arbitrarily close.

Moreover with b ≥ diam(X ), Θ, N as above:

∀ε > 0∀n ≥ ΩD,b,Θ(ε) (d(Ln+1,Mn) < ε),

whereΩD,b,Θ(ε) = N + N

Θ(N)(α)

⌉with

0 < α ≤ min1N,D2,ε

Theorem (K./Lopéz-Acedo/Nicolae 2019)

Let X be a bounded metric space with the uniform betweenness propertyand 〈(Mn), (Ln)〉 be an arbitrary Lion-Man game with speed D > 0.Then the Lion approaches the man arbitrarily close.

Moreover with b ≥ diam(X ), Θ, N as above:

∀ε > 0 ∀n ≥ ΩD,b,Θ(ε) (d(Ln+1,Mn) < ε),

whereΩD,b,Θ(ε) = N + N

Θ(N)(α)

⌉with

0 < α ≤ min1N,D2,ε

Uniform betweenness in normed spaces

Let (X , ‖ · ‖) be a normed space.

Proposition (Diminnie, White 1981)

The betweennes property (BW) is equivalent to(BW)′: for all x, y , z ∈ X

‖x‖ = ‖y‖ = ‖z‖ =

∥∥∥∥x + y2

∥∥∥∥ =

∥∥∥∥y + z2

∥∥∥∥ = 1→ ‖x + y + z‖ = 3.

(BW)′ also has an obvious uniformization (UBW)′: for all ε > 0 thereexists a δ > 0 such that for all x, y , z ∈ X with‖x‖ = ‖y‖ = ‖z‖ = 1:∥∥∥∥x + y

∥∥∥∥ , ∥∥∥∥y + z2

∥∥∥∥ ≥ 1− δ → ‖x + y + z‖ ≥ 3− ε

together with the corresponding concept of a modulusδ : (0,∞)→ (0,∞).

Uniform betweenness in normed spaces

Let (X , ‖ · ‖) be a normed space.

The betweennes property (BW) is equivalent to(BW)′: for all x, y , z ∈ X

‖x‖ = ‖y‖ = ‖z‖ =

∥∥∥∥x + y2

∥∥∥∥ =

∥∥∥∥y + z2

∥∥∥∥ = 1→ ‖x + y + z‖ = 3.

(BW)′ also has an obvious uniformization (UBW)′: for all ε > 0 thereexists a δ > 0 such that for all x, y , z ∈ X with‖x‖ = ‖y‖ = ‖z‖ = 1:∥∥∥∥x + y

∥∥∥∥ , ∥∥∥∥y + z2

∥∥∥∥ ≥ 1− δ → ‖x + y + z‖ ≥ 3− ε

together with the corresponding concept of a modulusδ : (0,∞)→ (0,∞).U. Kohlenbach (joint work with G. Lopéz-Acedo and A. Nicolae) Proof Mining and the ‘Lion-Man’ Game

Proposition (K.,Lopéz-Acedo,Nicolae 2019)

Let (X , ‖ · ‖) be a normed space. Then X satisfies (UBW) iff it satisfies(UBW)′. Moreover, respective moduli can be transformed into each otherby the transformations

Θ(ε, a, b) := 2a · δ(ε

), δ(ε) :=

2,12, 3),12,ε

Examples of uniquely geodesic spaceswith uniform betweenness

Definition (K./Lopéz-Acedo/Nicolae 2019)

We say that X is uniformly uniquely geodesic if for all ε, b > 0 thereexists ϕ > 0 such that for all x , y , z1, z2 ∈ X with d(x , y) ≤ b and allt ∈ [0, 1] we have

d(x, z1) ≤ td(x, y), d(y , z1) ≤ (1− t)d(x, y) + ϕ

d(x, z2) ≤ d(x, y), d(y , z2) ≤ (1− t)d(x, y) + ϕ

⇒d(z1, z2)<ε.

A mapping Φ : (0,∞)× (0,∞)→ (0,∞) providing for given ε, b > 0such a ϕ = Φ(ε, b) is called a modulus of uniform uniqueness.

Proposition (K.,Lopéz-Acedo,Nicolae 2019)

Let X be a uniformly uniquely geodesic space with modulus Φ whichsatisfies the convexity condition

d(z, (1− t)x + ty) ≤ (1− t)d(z, x) + td(z, y).

Θ(ε, a, b) = min

a · ε8b

, b), a

is a modulus of uniform betweenness.

Moduli Φ and hence Θ can be explicitly computed forLp (1 < p <∞) and CAT(κ)-spaces, κ > 0.

For Lp:

Φ(ε, b) =

p − 18

(b + ε), if 1 < p ≤ 2,

(b + ε)p−1, if 2 < p <∞.

For CAT(κ)-spaces X , κ > 0, with diam(X ) < π/(2√κ):

Φ(ε, b) =c16

b + ε, where

c = (π−2√κβ) tan(

√κβ) for any 0 < β ≤ π/(2

√κ)−diam(X ).

For Lp:

Φ(ε, b) =

p − 18

(b + ε), if 1 < p ≤ 2,

(b + ε)p−1, if 2 < p <∞.

Φ(ε, b) =c16

b + ε, where

√κ)−diam(X ).

For Lp:

Φ(ε, b) =

p − 18

(b + ε), if 1 < p ≤ 2,

(b + ε)p−1, if 2 < p <∞.

Φ(ε, b) =c16

b + ε, where

√κ)−diam(X ).

Examples of (nonuniquely)geodesic spaces with uniform

betweenness

Ptolemy spaces

Definition

A metric space (X , d) is a Ptolemy space if for all x, y , z,w ∈ X

d(x, z)d(y ,w) ≤ d(x, y)d(z,w) + d(x,w)d(y , z).

Proposition (Foertsch, Lytchak, Schroeder 2007)

There are complete bounded Ptolemy spaces which are geodesic but notuniquely geodesic.

Proposition (Nicolae 2013)

Every Ptolemy metric space satisfies the betweenness property.

Ptolemy spaces

Definition

Ptolemy spaces

Definition

Being Ptolemy is a purely universal axiom which, therefore, is admissibleto be used in uniform bound extraction theorems for metric spaces.Hence the extractability of a modulus Θ is guaranteed!

Indeed an easy analysis gives:

Proposition (K., Lopéz-Acedo, Nicolae 2019)

Let (X , d) be a Ptolemy space. Then Θ(ε, a, b) :=√

b2 + εa − b is amodulus for the uniform betweenness property.

Being Ptolemy is a purely universal axiom which, therefore, is admissibleto be used in uniform bound extraction theorems for metric spaces.Hence the extractability of a modulus Θ is guaranteed!

Indeed an easy analysis gives:

Let (X , d) be a Ptolemy space. Then Θ(ε, a, b) :=√

b2 + εa − b is amodulus for the uniform betweenness property.

A nonstrictly normed space with the uniform betweennessproperty

Definition (Diminnie, White 1981)

Consider R3 with the norm

‖(x, y , z)‖DW :=√|z2 − (x2 + y2)|+ 3z2 + x2 + y2.

(X , ‖ · ‖DW) is not strictly normed (and hence not uniquely geodesic) butsatisfies the betweenness property.

Guaranteed by logical bound extraction metatheorems (this time we usethat K := x ∈ R3 : ‖x‖DW ≤ b is compact): there must be amodulus for the uniform betweenness property extractable from the proof(by some affine shift we may assume that e.g. x := 0).

Indeed, the (this time complicated) logical analysis of the proof byDiminnie and White gives:

Let η(ε) := ε2/8 and 0 < ε ≤ 1/2.

Θ(ε, a, b) := 2a · δ(ε/2b) with

δ(ε) := minη(√

2·ε256

is a modulus for the uniform betweennes property of (X , ‖ · ‖DW).

Guaranteed by logical bound extraction metatheorems (this time we usethat K := x ∈ R3 : ‖x‖DW ≤ b is compact): there must be amodulus for the uniform betweenness property extractable from the proof(by some affine shift we may assume that e.g. x := 0).

Indeed, the (this time complicated) logical analysis of the proof byDiminnie and White gives:

Let η(ε) := ε2/8 and 0 < ε ≤ 1/2.

Θ(ε, a, b) := 2a · δ(ε/2b) with

δ(ε) := minη(√

2·ε256

is a modulus for the uniform betweennes property of (X , ‖ · ‖DW).

Literature

Diminnie, C.R., White, A.G., Remarks on strict convexity andbetweenness postulates, Demonstr. Math. 14, pp. 209-220 (1981).Kohlenbach, U., Recent progress in proof mining in nonlinearanalysis. IFCoLog Journal of Logics and their Applications 10, Issue4, pp. 3357-3406 (2017).Kohlenbach, U., Proof-theoretic Methods in Nonlinear Analysis. In:Proc. ICM 2018, Vol. 2, pp. 61-82. World Scientific 2019.Kohlenbach, U., López-Acedo, G., Nicolae, A., A uniformbetweenness property in metric spaces and its role in the quantitativeanalysis of the ‘Lion-Man’ game. To appear in: Pacific J. Math.Littlewood’s Miscellany (ed: B. Bollobás), Cambridge UniversityPress, Cambridge, 1986.López-Acedo, G., Nicolae, A., Piątek, B., ‘Lion-Man’ and the fixedpoint property, Geom. Dedicata 202, pp. 69-80 (2019).

Proof Mining and the `Lion-Man' Game · 2020. 12. 3. · Proof Mining and the ‘Lion-Man’ Game...

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