Large Deviations Principles for McKean-Vlasov SDEs, Skeletons, Supports and the law of iterated logarithm

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1 Large Deviations Principles for McKean-Vlasov SDEs, Skeletons, Supports and the law of iterated logarithm Gonçalo dos Reis University of Edinburgh (UK) & CMA/FCT/UNL (PT) jointly with: W. Salkeld, U. of Edinburgh (UK) J. Tugaut, U. Jean Monnet (FR) LMS/EPSRC Durham Symposium in Stochastic Analysis Durham, 17 July 2017 Partial funding by UID/MAT/00297/2013 Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

2 Overview 1 McKean Vlasov Equations 2 Large Deviations Principles Skeleton ODE s of SDE s LDPs Results 3 Applications Functional Strassen s law 4 Outlook and Further Extensions Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

3 Outline 1 McKean Vlasov Equations 2 Large Deviations Principles Skeleton ODE s of SDE s LDPs Results 3 Applications Functional Strassen s law 4 Outlook and Further Extensions Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

4 McKean-Vlasov Stochastic Differential Equations Definition A McKean-Vlasov SDEs (MV-SDE) is an SDE where the coefficients are dependent on the law L(X ( )) of the solution process (X ( )). We write ( ) ( ) dx (t) = b t, X (t), L(X (t)) dt + σ t, X (t), L(X (t)) dw (t) Example (Mean Field Scalar Interaction) Let us consider a simple example: t [ ] X (t) = x + E[X (s)] X (s) ds + W (t) 0 X (0) = x Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

5 McKean-Vlasov Stochastic Differential Equations Definition A McKean-Vlasov SDEs (MV-SDE) is an SDE where the coefficients are dependent on the law L(X ( )) of the solution process (X ( )). We write ( ) ( ) dx (t) = b t, X (t), L(X (t)) dt + σ t, X (t), L(X (t)) dw (t) Example (Mean Field Scalar Interaction) Let us consider a simple example: t [ ] X (t) = x + E[X (s)] X (s) ds + W (t) 0 Question: Is this an standard SDE after decoupling? X (0) = x Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

6 Distributions and the Wasserstein Metric Definition Let (E, d) be a Polish space and σ-algebra E. Let P 2 (E) be the space of probability distributions on (E, E) with finite second moments. Let µ, ν P 2 (E). We define the Wasserstein distance to be {( ) 1/2; } W (2) (µ, ν) = inf d(x, y) 2 π(dx, dy) π P(E E) E 2 where µ(a) = E 2 χ A (x)π(dx, dy) and ν(b) = E 2 χ B (y)π(dx, dy). See [Carmona, 2016] form more details. Example The Wasserstein distance between the law of a RV X and a constant y is W (2) (L(X ), δ y ) = E[ X y 2 ] 1/2 Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

7 Existence and Uniqueness Theorem (Existence and uniqueness) Let (X (t)) t 0 satisfy the MV-SDE dx (t) = b(t, X (t), L(X (t)))dt + σ(t, X (t), L(X (t)))dw (t), X (0) µ 0 ( P 2 P 4 ) (R d ) with: L > 0, K R t [0, T ], x, x R d, µ, µ P 2 (R d ) s.th. ( ) σ(t, x, µ) σ(t, x, µ ) L x x + W (2) (µ, µ ) x y, b(t, x, µ) b(t, y, µ) R d K x y 2 b(t, x, µ) b(t, x, µ ) LW (2) (µ, µ ) Then there exists a unique solution X and C > 0 such that [ E X (t) 2] ( ) E[ X (0) 2 ] + C e CT sup t [0,T ] Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

8 Properties Theorem (Properties) Integrability 1 p > 1 we have E[sup t X (t) p ] < (with agreeing integrability of X (0), b(, 0, δ 0 ), σ(, 0, δ 0 )) Continuity 1 paths of t X (t)(ω) are a.s. continuous in C α, α < 1/2. 2 t L(X (t)) is C 1/2 in the W (p) -metric Differentiability 1 for any p 1 the map t E[ X p (t) p ] C 1 2 Malliavin differentiability: X D 1,2 (with deterministic coefficients) Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

9 Outline 1 McKean Vlasov Equations 2 Large Deviations Principles Skeleton ODE s of SDE s LDPs Results 3 Applications Functional Strassen s law 4 Outlook and Further Extensions Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

10 Deterministic Approximation of SDE Definition Let H be the Cameron Martin space, the space of all absolutely continuous paths h(t) = t 0 ḣ(s)ds such that ḣ L2 ([0, 1]). Definition We approximate the McKean Vlasov SDE by the ODE dx (t) = b ε (t, X (t), L(X (t)))dt + εσ(t, X (t), L(X (t)))dw (t) X (0) = x dφ(h)(t) = b(t, Φ(h)(t), δ Φ(t) )dt + σ(t, Φ(h)(t), δ Φ(t) )ḣ(t)dt and we call this the Skeleton. Φ(0) = x Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

11 Deterministic Appoximation of SDE Example The SDE t [ [ X (t) = x + X (s) E X (s) 3]] ds + εw (t) 0 has a Skeleton h H Φ(h)(t) = x + since Ω x 3 dδ x (y) = y 3. t 0 [ Φ(h)(s) Φ(h)(s) 3] ds + h(t) Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

12 Large Deviations Principles Definition (Large Deviations Principle) Let (E, d) be a Polish space and let {P N } N N be a sequence of Borel probability measures on E. Let I : E [0, ] be a lower semicontinuous functional on E. The sequence {P N } N N is said to satisfy a Large Deviations Principle with rate function I log(p N [A]) log(p N [A]) inf I (x) lim inf x Å N N 2 lim sup N N 2 inf I (x) x A for any Borel measurable set A E. Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

13 LDP for Brownian Motion Consider the following simple example for Brownian motion with a supremum norm. Example (LDP in uniform Norm for BM) Consider the simple example of (εb t ) t. ] We know for R >> 1 fixed that P[ εb > R e R2 /(2ε 2). Therefore (apply log + scalling & limits) ( lim sup ε 2 ]) log P[ εb > R ε 0 lim ε 2 c R2 ε 0 2 = R2 2 The rate function for Brownian motion would output R2 2 for the set {x(t) C([0, 1]) : x > R} the set of continuous paths starting at 0 such that the supremum of the path is greater than R. Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

14 LDP for MV-SDEs Our goals: 1 an -topology LDP for X (see [Gärtner, 1988], [Budhiraja et al, 2012]) 2 a conditional α -topology type LDP for X, Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

15 LDP for MV-SDEs Our goals: 1 an -topology LDP for X (see [Gärtner, 1988], [Budhiraja et al, 2012]) 2 a conditional α -topology type LDP for X, Theorem R, ρ > 0, δ, ν > 0 such that 0 < ε < ν, [ ] P Xε x Φ x (0) α ρ, εw δ exp( R/ε 2 ) Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

16 Hölder Norms using Ciesielski Isomorphism Definition Let H 00 (t) = 1 and 2 p, H pm (t) = 2 p, 0, otherwise. if t [ m 1 2, 2m 1 ), p 2 p+1 if t [ 2m 1 2 p+1, m 2 p ), where m {1,..., 2 p } and p N {0}. These are called the Haar functions. Figure: Haar Function H pm (t) Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

17 Hölder Norms using Ciesielski Isomorphism Ciesielski s Isomorphism Define the Fourier coefficients ψ pm = 1 0 H pm(s)ψ(s)ds, ψ pm := H pm, dψ := ( 2m 1 ) ( m 1 2 [2ψ p 2 p+1 ψ additionally ψ 00 := H 00, dψ = ψ(1) ψ(0). Let G pm (t) = t 0 H pm(s)ds. Then ψ(t) = ψ 00 G 00 (t) + 2 p p=0 m=1 2 p ) ψ pm G pm (t) ( m )] ψ 2 p, Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

18 Hölder Norms using Ciesielski Isomorphism The Hölder Norm The Hölder Norm is defined to be f α = f (0) + sup t,s [0,1] f (t) f (s) t s α We have that α is equivalent to (see [Ciesielsky, 1960]) ψ α = sup 2 (α 1/2)p ψ pm. p,m Throughout this talk, we will assume that α < 0.5. Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

19 Auxilliary Lemmas Lemma 1 C > 0 such that u > 0 and for all processes K on [0, 1] we have Lemma 2 [ ] P K(s)dW (s) u, K 1 C exp( u 2 /C) α 0 C > 0 such that u, v > 0 we have [ ] ( ( u ) 1/α ) ( 1 P W α u, W v C u 1/α ) max 1, exp v C v 1/α 2. These are proved via the equivalence of norms from Ciesielski s isomorphism + Chernoff s inequality. Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

20 Main Results Definition Let h H be an element of the Cameron Martin Space, σ bdd. We consider the SDE X x ε = x + + ε t 0 t 0 b ε (s, X x ε (s), L(X x ε (s)))ds σ ε (s, X x ε (s), L(X x ε (s)))dw (s) with Skeleton (b ε b, σ ε σ uniformly as ε 0) t Φ x (h)(t) = x t 0 b(s, Φ x (h)(s), δ Φ x (h)(s))ds σ(s, Φ x (h)(s), δ Φ x (h)(s))ḣ(s)ds Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

21 Main Results Theorem R, ρ > 0, δ, ν > 0 such that 0 < ε < ν, [ ] P Xε x Φ x (h) α ρ, εw h δ exp( R/ε 2 ) Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

22 Main Results Theorem R, ρ > 0, δ, ν > 0 such that 0 < ε < ν, [ ] P Xε x Φ x (h) α ρ, εw h δ exp( R/ε 2 ) From the above inequality follows Theorem Let A be a Borel set of the space of R-valued continuous paths over [0, 1] in the Hölder topology. Let (A) := inf { ḣ 2 2 /4; h H, Φx (h)( ) A }. Then (Å) lim inf ε 0 ε 2 2 log P[X ε x ε 2 A] lim sup ε 0 2 log P[X ε x A] (Ā) where Å and Ā are the interior and closure of the set A with respect to the topology generated by the Hölder norm. Our proof follows loosely the methods of [Arous, 1994]. Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

23 Proof of Main Result Proof. We condition on the event that the process Xε x (t) remains in the ball of radius N and we see [ ] P Xε x Φ x (h) α ρ, εw h δ [ ] [ ] P Xε x Φ x (h) α ρ, εw h δ, Xε x < N + P Xε x N We use that we have the LDP result for Xε x in a supremum norm and choose N large enough so that [ ] ( P Xε x N < exp N ) ε 2. (We give & prove LDP in -topology, we do not state it here.) Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

24 Proof of Main Result Proof. Let Xε x,l be a step function approximation of Xε x. [ P ε [ P ε 0 ] σ ε (s, Xε x (s), L(Xε x (s)))dw (s) α ρ, εw δ, Xε x < N 0 [σ ε (s, X x ε (s), L(X x ε (s))) σ ε ( sl l 1 + X x l β ε Xε x,l + P[ 1 + X x l β ε Xε x,l [ + P ε σ ε ( sl l, Xε x,l 0 exp( R ε 2 ), X x,l ε + E[ Xε x Xε x,l 2 ] 1/2 γ, L(Xε x ( sl l )))]dw (s) α ρ 2, ] + E[ Xε x Xε x,l 2 ] 1/2 > γ, Xε x < N (s), L(Xε x ( sl l )))dw (s) α ρ 2, εw δ ] ] Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

25 Outline 1 McKean Vlasov Equations 2 Large Deviations Principles Skeleton ODE s of SDE s LDPs Results 3 Applications Functional Strassen s law 4 Outlook and Further Extensions Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

26 Strassens Law for Brownian Motion Theorem Let W (t) be a Brownian Motion. Then X n (t) = W (nt) n Y n (t) = W (nt) n X n is a Brownian motion but Y n converges almost surely to 0 as n. Strassens Law states that Z n (t) = W (nt) n log(log(n)) converges to 0 in probability but does not converge almost surely. Therefore we get the well known result lim sup n W (n) n log(log(n)) = 2 Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

27 Contraction Operators Definition Let α R +. A family of continuous bijections Γ α : R d R d is said to be a System of Contractions centered at x if 1 Γ α (x) = x for every α R +. 2 If α β then Γ α (y 1 ) Γ α (y 2 ) Γ α (z 1 )+Γ α (z 2 ) Γ β (y 1 ) Γ β (y 2 ) Γ β (z 1 )+Γ β (z 2 ) for every y 1, y 2, z 1, z 2 R d. 3 Γ 1 is the identity and (Γ α ) 1 = Γ α 1. 4 For every compact set K C α ([0, 1]; R d ), f K and ε > 0, δ > 0 such that pq 1 < δ implies Γ p Γ q (f ) f α < ε, p, q R +. Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

28 Law of Iterated Logarithms for McKean Vlasov SDEs Definition Let Y be the solution to the SDE dy (t) = b(y (t), L(Y (t)))dt + σ(y (t), L(Y (t)))dw (t), Y (0) = x R d Denote φ(u) = u log(log(u)). Consider the coefficients ]( ) T ) ˆσ u (y, µ) = φ(u) [Γ φ(u) Γ φ(u) 1(y) σ (Γ φ(u) 1(y), µ Γ φ(u) ]( ) ˆb u (y, µ) = ul(y, µ) [Γ φ(u) Γ φ(u) 1(y) where the operator (with ã = σ T σ) [ ]( ) d f ( ) L(y, µ) f z = Γ y φ(u) 1(z) )b i (Γ φ(u) 1(y), µ Γ φ(u) i i= d i,j=1 ( ) 2 f ( ) ã i,j Γ φ(u) 1(y), µ Γ φ(u) Γ y i y φ(u) 1(z). j Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

29 Law of Iterated Logarithms for McKean Vlasov SDEs Assumption Assume that (ˆσ u, ˆb u ) (ˆσ, ˆb) uniformly on R d P 2 (R d ) as u. Further, assume that ˆσ(y, µ) is bounded and Lipschitz and that ˆb(y, µ) has monotone growth in y and is Lipschitz in µ. Definition [ ] Let Z u (t) = Γ φ(u) Y (ut) and note that Wu (t) = W (ut) u is a Brownian motion ) ˆσ u (Z u (t), L(Z u (t)) ) dz u (t) = dw u (t) + ˆb u (Z u (t), L(Z u (t)) dt log log(u) with Skeleton ( ) dφ(h)(t) = ˆσ Φ(h)(t), δ Φ(h)(t) )ḣ(t)dt + ˆb (Φ(h)(t), δ Φ(h)(t) dt Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

30 Law of Iterated Logarithms for McKean Vlasov SDEs α < 1/2 Theorem With probability 1, the set of paths {Z u ; u > 3} is relatively compact in the Hölder topology C α and its set of limit points coincides with K = {Φ(h) : ḣ }. Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

31 Proof of the Law of Iterated Logarithms Proof We prove 2 Propositions: 1 Relatively compact. For every ε > 0 there exists a.s. a positive real number u 0 (ω) such that for every u > u 0 d α (Z u (ω), K) < ε where for x C α ([0, 1]) and M C α ([0, 1]) d α (x, M) = inf y M x y α 2 Limit point. Let g K. Then ε > 0, c ε > 1 such that c > c ε [ ] P Z c j g α < ε for j i.o. = 1 Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

32 Proof of the Law of Iterated Logarithms Proof of Proposition (1) - Relative compactness To prove the first Proposition, we argue (c > 1, j N and j >> 1) d α (Z u, K) d α (Z c j, K) Then we use the following Lemma + Γ φ(u) Γ 1 φ(c j ) (Z c j ) Z c j α + Z u Γ φ(u) Γ φ(c j ) 1(Z c j ) α Lemma c > 1, ε > 0 then there exists a.s. j 0 (ω) N such that j > j 0 d α (Z c j, K) < ε Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

33 Proof of Auxilliary Lemmas Proof of Lemma Let K ε = {g; d α (g, K) > ε}. Then δ > 0 such that (K ε) > 1 + 2δ. [ P Z c j K ε [ Hence by Borel Cantelli P Z c j ] ( ) exp (1 + δ) log log(c j ) 1 ] K ε for j i.o. = 0. j 1+δ Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

34 Proof of Law of Iterated Logarithms Proof of Proposition (2) - The limit Points Let h H s.th. h Φ(h) K. Define (recall W u (t) = W (ut)/ u) { } { } W E j = c j (t) Zc log log(c j ) h β and F j = j Φ(h) ε α By the Hölder topology LDP: [ ] P[E j ] P[F j ] = P E j Fj c ( ) exp 2 log log(c j ) 1 j 2 However, we also have ] P [E j =, j j ( ) P[E j ] P[F j ] < j ] P [F j =. Hence [ ] P Z c j Φ(h) α < ε i.o. = 1. Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

35 Outline 1 McKean Vlasov Equations 2 Large Deviations Principles Skeleton ODE s of SDE s LDPs Results 3 Applications Functional Strassen s law 4 Outlook and Further Extensions Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

36 Outlook and Further Extensions Takeway... Existence & uniqueness results, regularity, LDPs in path space, Iterated logarithm law Techniques able to directly deal with the MV-SDE law Outlook (Topological characterization) Support Theorem for MV-SDEs Existence and uniqueness for the fully coupled case Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

37 References Baldi. P (1986) Large Deviations and Functional Iterated Logarithm Law for Diffusion Processes Probability Theory and Related Fields (Springer) Ben Arous, G. and Ledoux, Michel Grandes déviations de Freidlin-Wentzell en norme Hölderienne Springer Carmona, René (2016) Lectures of BSDEs, Stochastic Control, and Stochastic Differential Games with Financial Applications SIAM Ciesielski. Z (1960) On the Isomorphisms of the Spaces H α and m BULETIN DE L ACADÉMIE POLONAISE DES SCIENCES Gärtner, J. (1988) On the McKean-Vlasov Limit for Interacting Diffusions Math. Nachr. 137 Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

38 Thank you Goncalo dos Reis (U. of Edin. & CMA) LDP for McKean Vlasov SDE s Durham Symp., 17 Jul / 35

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