A pointwise equivalence of gradients on configuration spaces

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1 A pointwise equivalence of gradients on configuration spaces Nicolas Privault Equipe d Analyse et Probabilités, Université d Evry-Val d Essonne Boulevard F. Mitterrand, Evry Cedex, France privault@lami.univ-evry.fr Abstract - This Note aims to provide a short and self-contained proof of an equivalence between square field operators on configuration spaces. A recent result in analysis on configuration spaces, namely an equivalence between Dirichlet forms, is retrieved as a particular case. Our method relies on duality formulas and on extensions of the stochastic integral on Poisson space. Une équivalence de gradients ponctuelle sur les espaces de configurations Résumé - Cette Note donne une preuve concise d une équivalence entre opérateurs carré du champ sur l espace des configurations. Ceci permet de retrouver un résultat récent relatif à une équivalence entre formes de Dirichlet sur ce même espace. La méthode utilisée repose sur des formules de dualité et sur les extensions de l intégrale stochastique sur l espace de Poisson. Version française abrégée L analyse sur les espaces de configurations développée dans 1] repose sur une égalité de normes en espérance pour le gradient local Υ et l opérateur de différence finie D sur les espaces des configurations Υ sur une variété Riemannienne. Dans cette Note nous présentons une version ponctuelle de cette relation, avec une preuve concise qui fait apparaître le rôle de la dualité sur l espace de Poisson. On montre que ( Υ F, Υ G L 2 γ (T = δ ( ( DF, DG T + ( DF, DG L 2 σ (T (1 pour γ Υ et F, G S, où δ est l adjoint de l opérateur de différence finie D sous la mesure de Poisson π σ d intensité σ sur Υ. Cette relation s écrit ( Υ F, Υ G L 2 γ (T = ( DF, ] DG Tx (γ\{x}dγ(x, où γ\{x} représente la configuration γ Υ à laquelle on a enlevé le point x. En prenant l espérance dans (1 on obtient comme corollaire l égalité ] σ ( Υ F, Υ G L 2 γ (T = σ ( DF, DG L 2 σ (T ], pour toutes fonctionnelles F et G régulières sur Υ, c est à dire le Th. 5-2 de 1], et ce uniquement sous la mesure de Poisson π σ. Plus généralement on montre que l opérateur carré du champ associé à la seconde quantification différentielle d un opérateur de diffusion conservatif H sur Cc (, de carré du champ local Γ, est donné par Γ Υ (F, G = δ ( Γ (DF, DG + Γ (DF, DGdσ, F, G S. (2 1

2 1 Introduction The study of the stochastic calculus of variations for point processes started in 3], 2], has been developed in several different directions, using mainly two different gradients. The gradient on Fock space is interpreted as a difference operator and has been used in e.g. 7], 9], 11]. On the other hand, infinitesimal perturbations of configurations, cf. 3], 2], 6], 4], 1], lead to a gradient that defines a local Dirichlet form. Stochastic analysis on configuration spaces, cf. 1], relies on the equivalence of expectation norms (13 that links the local gradient Υ to the finite difference operator D on the configuration space Υ of a Riemannian manifold. In this paper we use the duality on Poisson space to obtain a pointwise equivalence of gradients. 2 Difference operators on configuration spaces The results of this section and the following hold without referring to a particular probability measure on Υ. Let be a metric space and let the configuration space Υ on be the set of Radon measures on of the form i=n ε x i, with (x i i=n, x i x j, i j, and N IN { }. Let σ be a diffuse Borel measure on. As a convention we identify γ Υ with its support. Following 10], for any x and any mapping F : Υ IR we define the mappings ε + x F : Υ IR and ε x F : Υ IR by ε x F ] (γ = F (γ\{x}, and ε + x F ] (γ = F (γ {x}, γ Υ. The difference operator D has been defined in 10] as D x F = ε + x F ε x F, x. For u : Υ IR measurable (and everywhere defined, the negative and positive Skorohod integral operators are defined as δ (u = ε x u x d(γ(x σ(x = ε x u x dγ(x u x dσ(x, (3 and δ + (u = ε + x u x (dγ(x dσ(x = u x dγ(x ε + x u x dσ(x, (4 γ Υ, provided the series and integrals converge. The proof of the following lemma is straightforward from (3 and (4. Lemma 1 For u : Υ IR measurable, the operators δ, δ + and D are linked to the anticipating pathwise integral by the identities u x d(γ(x σ(x = δ + (u + D x u x dσ(x, (5 and provided that all series and integrals converge. u x d(γ(x σ(x = δ (u + D x u x dγ(x, (6 With respect to multiple integrals it is well known that D acts by annihilation and δ acts by creation. This property does not hold for δ +, in fact δ + (u = δ (u + Du. 2

3 3 Differential operators on configuration spaces We now assume that is a Riemannian manifold with volume element m, and σ(dx = ρ(xm(dx with ρ L 2 loc (, m. Let T x denote the tangent space at x, let denote the gradient on, and let L 2 γ(t = L 2 (, T, γ, resp. L 2 σ(t = L 2 (, T, σ denote the random, resp. deterministic tangent space to Υ at γ Υ, cf. 1]. Let also S = { f ( ϕ 1 dγ,..., ϕ n dγ, ϕ 1,..., ϕ n C c (, f C b (IR n, n IN where P(IR n denotes the space of real polynomials in n variables, and { i=n } U = F i u i : u 1,..., u n Cc (, F 1,..., F n S, n 1. The following gradient has been defined for = IR d in 2], p. manifold in 1]. Definition 1 For any F = f ( ϕ 1dγ,..., ϕ ndγ S, let i=n ( Υ x F (γ = i f ϕ 1 dγ,..., ϕ n dγ ϕ i (x, x. }, 152, and for a Riemannian The following result gives the relation between Υ, D and the gradient on. Lemma 2 We have for F S and γ Υ : Υ x F = ε x D x F, γ(dx a.e. and ε + x Υ x F = D x F, σ(dx a.e. Proof. Let F = f ( ϕ 1dγ,..., ϕ ndγ, x, γ Υ. If x γ, then i=n ( Υ x F (γ = i f ϕ 1 d(γ\{x} + ϕ 1 (x,..., ϕ n d(γ\{x} + ϕ n (x ϕ i (x ( = f ϕ 1 d(γ\{x} + ϕ 1 (x,..., ϕ n d(γ\{x} + ϕ n (x ( = ε + x f ϕ 1 d(γ\{x},..., ϕ n d(γ\{x} = ( ε + x F (γ\{x} = ( D x F (γ\{x}, γ(dx a.e. If x / γ, hence σ(dx-a.e., we have ( ε + x Υ x F (γ = f ϕ 1 dγ + ϕ 1 (x,..., ( = ε + x f ϕ 1 dγ,..., ϕ n dγ Lemma 2 and (3, (4, imply the following ϕ n dγ + ϕ n (x Proposition 1 For F, G S and any γ Υ, we have the relations and = ε + x F (γ = D x F (γ. ( Υ F, Υ G L 2 γ (T = δ ( ( DF, DG T + ( DF, DG L 2 σ (T, (7 ( Υ F, Υ G L 2 γ (T = δ + ( ( Υ F, Υ G T + ( DF, DG L 2 σ (T. (8 3

4 4 Integration by parts characterization Theorem 1 Let π be a probability measure on Υ under which δ (h, resp. δ + (h, is integrable, h C c (. The following statements are equivalent, and they hold if and only if π is the Poisson measure π σ with intensity σ: F δ (h ] = (DF, hl 2 (,σ], h Cc (, F S, (9 F δ + (h ] = (DF, hl2 (,γ], h Cc (, F S, (10 δ + (u ] = 0, u U, (11 δ (u ] = 0, u U. (12 Proof. Clearly, these statements are equivalent, due to Lemma 1 applied to u = hf, and to the relations δ + (u = δ (ε + u, δ (u = δ + (ε u, u U. Assume that (9 holds, let h C c ( and ψ(z = exp ( iz hdγ ], z IR. We have d dz ψ(z = i = i δ (h exp ( hdγ exp ( iz ( ( = i h, D exp iz = i(h, e izh 1 L2 (,σ exp ] iz hdγ ] hdγ + i ] hdγ ( iz L 2 (,σ hdγ ( hdσ exp iz + iψ(z hdσ ] + iψ(z ] hdγ hdσ = iψ(z(h, e izh L2 (,σ, where we used the relation D x exp( hdγ = 1 exp( hdγ, σ(dx-a.e. With the initial (ehx condition ψ(0 = 1, this gives ψ(z = exp (eizh 1dσ, z IR. Conversely, this calculation shows by completeness of exponential functions that (9 is satisfied if π = π σ. Relations (11 and (12 can be regarded as the characterization results of 8], used respectively in 12] and 1]. Relation (9, resp. (10, has been obtained in 7], 9], resp. in 10], under the assumption π = π σ. As a consequence, Th. 5-2 of 1] is obtained by taking expectations under π σ in (7 or (8: ] σ ( Υ F, Υ G L 2 γ (T = σ ( DF, DG L 2 σ (T ], (13 hence the generator of the diffusion process associated to the Dirichlet form given by Υ is the differential second quantization dγ(h σ of the Dirichlet operator H σ = div σ, cf. 1], and 14], 2] for the construction of this process. Remark 1 Although (8 gives (13 by taking expectations, it does not yield any information as a random identity. On the other hand, (7 is a true pointwise identity that also reads ( Υ F, Υ G L 2 γ (T = ( DF, ] DG Tx (γ\{x}dγ(x, γ Υ. 4

5 5 Second quantization and carré du champ operators In this section we work in a more general setting and show that (13 is a consequence of the computation of the carré du champ operator associated to second quantized diffusion operators. The difference with the above construction is that we are not restricted to the structure given by. We refer to 13] and 5] for the definition of diffusion operators, carré du champ operators and their locality property on the Wiener and Poisson spaces as well as on more general structures. The differential second quantization dγ(h of an operator H on C c ( is defined as dγ(h F = δ (H DF, F S. The following theorem shows how to compute the carré du champ of dγ(h. Theorem 2 Let H be a diffusion operator on Cc (, conservative under σ, with carré du champ Γ. Then dγ(h is a diffusion operator with carré du champ Γ Υ (F, G = δ ( Γ (DF, DG + Γ (DF, DGdσ, F, G S. (14 Proof. Defining Γ Υ (F, G by (14 we have, using (5 and the relations δ + (u = δ (u + Du, DF 2 = 2F DF + DF DF σ-a.e.: 1 2 dγ(h (F 2 = 1 ( 2 δ H D(F 2 = δ ( F H DF + H (DF DF = F dγ(h F (DF, H DF L 2 (,σ + δ ( Γ (DF, DF = F dγ(h F + Γ Υ (F, F. It remains to prove that Γ Υ is local. We have δ ( Γ (DF 2, DG + Γ (DF 2, DGdσ = 2δ ( F Γ (DF, DG +δ ( Γ (DF DF, DG + 2F Γ (DF, DGdσ + Γ (DF DF, DGdσ = 2δ ( F Γ (DF, DG + 2δ ( DF Γ (DF, DG +2F Γ (DF, DGdσ + 2(DF, Γ (DF, DG L2 (,σ = 2F Γ (DF, DG. Relation (7 and Th. 2 show in particular that (F, G ( Υ F, Υ G L 2 γ (T is the carré du champ operator associated to the differential second quantization dγ(h σ of H σ. 6 Stochastic integration We now work with the generator Hσ and carré du champ Γ σ given by and study the consequences of Th. 2 on stochastic integration (Hσ is assumed to be conservative. We state a definition of adaptedness that does not require an ordering on, following the Wiener space construction of 15]. Let V = { u : u U}. A vector u = i=n F i u i V is said to be adapted if u i DF i = 0 on Υ, i = 1,..., n. The set of square-integrable adapted vectors, denoted by L 2 ad (Υ, T is the completion in L 2 (Υ, π σ L 2 (, T, σ of the adapted vectors in V. For x we define the bilinear form trace x on T x T x by trace x u v = (u, v Tx, u v T x T x. Let π be a probability on Υ under which Υ admits an adjoint on S V, denoted by div Υ π, and such that S is dense in L 2 (Υ, π. 5

6 Proposition 2 We have for u = i=n G i u i V: i=n div Υ π u = G i div Υ π u i trace x Υ x u(xγ(dx. (15 If π is the Poisson measure π σ with intensity σ, then div Υ π σ u = div σ u(xγ(dx trace x Υ x u(xγ(dx, u U, (16 and for any u L 2 ad (Υ, T, div Υ π σ (u = δ (div σ u = div σ u(xγ(dx. (17 Proof. Relation (15 follows from the derivation property of Υ. Relation (16 holds because div Υ π σ u i = δ (div σ u i = div σ u i dγ from (13. Relation (17 follows from (16 and the definition of adaptedness, using a density argument and the Itô isometry. References 1] S. Albeverio, Yu. G. Kondratiev, and M. Röckner. Analysis and geometry on configuration spaces. J. Funct. Anal., 154: , ] K. Bichteler, J.B. Gravereaux, and J. Jacod. Malliavin Calculus for Processes with Jumps, volume 2 of Stochastics Monographs. Gordon and Breach, ] J.M. Bismut. Calcul des variations stochastique et processus de sauts. Zeitschrift für Wahrscheinlichkeitstheories Verw. Gebiete, 63: , ] N. Bouleau. Constructions of Dirichlet structures. In J. Kral et al., editor, Potential theory ICPT 94, Kouty, Czech Republic, August 13 20, 1994, pages de Gruyter, ] N. Bouleau and F. Hirsch. Dirichlet Forms and Analysis on Wiener Space. de Gruyter, ] E. Carlen and E. Pardoux. Differential calculus and integration by parts on Poisson space. In Stochastics, Algebra and Analysis in Classical and Quantum Dynamics, pages Kluwer, ] A. Dermoune, P. Krée, and L. Wu. Calcul stochastique non adapté par rapport à la mesure de Poisson. In Séminaire de Probabilités II, volume 1321 of Lecture Notes in Mathematics. Springer Verlag, ] J. Mecke. Stationäre zufällige Masse auf lokalkompakten Abelschen Gruppen. Z. Wahrscheinlichkeitstheorie Verw. Geb., 9:36 58, ] D. Nualart and J. Vives. A duality formula on the Poisson space and some applications. In E. Bolthausen, M. Dozzi, and F. Russo, editors, Seminar on Stochastic Analysis, Random Fields and Applications, Progress in Probability, pages , Ascona, Birkäuser. 10] J. Picard. Formules de dualité sur l espace de Poisson. Ann. Inst. H. Poincaré Probab. Statist., 32(4: , ] J. Picard. On the existence of smooth densities for jump processes. Probab. Theory Related Fields, 105: , ] S. Roelly and H. Zessin. Une caractérisation de champs gibbsiens sur un espace de trajectoires. C. R. Acad. Sci. Paris Sér. I Math., 321: , ] D. Stroock. The Malliavin calculus, a functional analytic approach. J. Funct. Anal., 44: , ] D. Surgailis. On multiple Poisson stochastic integrals and associated Markov semi-groups. Probability and Mathematical Statistics, 3: , ] A.S. Üstünel. Construction du calcul stochastique sur un espace de Wiener abstrait. C. R. Acad. Sci. Paris Sér. I Math., 305: ,