Mathematical Surveys and Monographs Volume 164
Differentiable Measures and the Malliavin Calculus
Vladimir I. Bogachev
American Mathematical Society http://dx.doi.org/10.1090/surv/164
Differentiable Measures Differentiable Measures and the Malliavin Calculus and the Malliavin Calculus
Vladimir I. Bogachev
Mathematical Surveys and Monographs Volume 164
Differentiable Measures and the Malliavin Calculus
Vladimir I. Bogachev
American Mathematical Society Providence, Rhode Island EDITORIAL COMMITTEE Ralph L. Cohen, Chair MichaelA.Singer Eric M. Friedlander Benjamin Sudakov MichaelI.Weinstein
2010 Mathematics Subject Classification. Primary 28Cxx, 46Gxx, 58Bxx, 60Bxx, 60Hxx.
For additional information and updates on this book, visit www.ams.org/bookpages/surv-164
Library of Congress Cataloging-in-Publication Data Bogachev, V. I. (Vladimir Igorevich), 1961– Differentiable measures and the Malliavin calculus / Vladimir I. Bogachev p. cm. – (Mathematical surveys and monographs ; v. 164) Includes bibliographical references and index. ISBN 978-0-8218-4993-4 (alk. paper) 1. Measure theory. 2. Theory of distributions (Functional analysis) 3. Sobolev spaces. 4. Malliavin calculus. I. Title.
QA312.B638 2010 515.42–dc22 2010005829
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Preface ix
Chapter 1. Background material 1 1.1. Functional analysis 1 1.2. Measures on topological spaces 6 1.3. Conditional measures 19 1.4. Gaussian measures 23 1.5. Stochastic integrals 29 1.6. Comments and exercises 35
Chapter 2. Sobolev spaces on IRn 39 2.1. The Sobolev classes W p,k 39 2.2. Embedding theorems for Sobolev classes 45 2.3. The classes BV 51 2.4. Approximate differentiability and Jacobians 52 2.5. Restrictions and extensions 56 2.6. Weighted Sobolev classes 58 2.7. Fractional Sobolev classes 65 2.8. Comments and exercises 66
Chapter 3. Differentiable measures on linear spaces 69 3.1. Directional differentiability 69 3.2. Properties of continuous measures 73 3.3. Properties of differentiable measures 76 3.4. Differentiable measures on IRn 82 3.5. Characterization by conditional measures 88 3.6. Skorohod differentiability 91 3.7. Higher order differentiability 100 3.8. Convergence of differentiable measures 101 3.9. Comments and exercises 103
Chapter 4. Some classes of differentiable measures 105 4.1. Product measures 105 4.2. Gaussian and stable measures 107 4.3. Convex measures 112 4.4. Distributions of random processes 122
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4.5. Gibbs measures and mixtures of measures 127 4.6. Comments and exercises 131 Chapter 5. Subspaces of differentiability of measures 133 5.1. Geometry of subspaces of differentiability 133 5.2. Examples 136 5.3. Disposition of subspaces of differentiability 141 5.4. Differentiability along subspaces 149 5.5. Comments and exercises 155 Chapter 6. Integration by parts and logarithmic derivatives 157 6.1. Integration by parts formulae 157 6.2. Integrability of logarithmic derivatives 161 6.3. Differentiability of logarithmic derivatives 168 6.4. Quasi-invariance and differentiability 170 6.5. Convex functions 173 6.6. Derivatives along vector fields 180 6.7. Local logarithmic derivatives 183 6.8. Comments and exercises 187 Chapter 7. Logarithmic gradients 189 7.1. Rigged Hilbert spaces 189 7.2. Definition of logarithmic gradient 190 7.3. Connections with vector measures 194 7.4. Existence of logarithmic gradients 198 7.5. Measures with given logarithmic gradients 204 7.6. Uniqueness problems 209 7.7. Symmetries of measures and logarithmic gradients 217 7.8. Mappings and equations connected with logarithmic gradients 222 7.9. Comments and exercises 223 Chapter 8. Sobolev classes on infinite dimensional spaces 227 8.1. The classes W p,r 227 8.2. The classes Dp,r 230 8.3. Generalized derivatives and the classes Gp,r 233 8.4. The semigroup approach 235 8.5. The Gaussian case 239 8.6. The interpolation approach 242 8.7. Connections between different definitions 246 8.8. The logarithmic Sobolev inequality 250 8.9. Compactness in Sobolev classes 253 8.10. Divergence 254 8.11. An approach via stochastic integrals 257 8.12. Some identities of the Malliavin calculus 263 8.13. Sobolev capacities 265 8.14. Comments and exercises 274 Contents vii
Chapter 9. The Malliavin calculus 279 9.1. General scheme 279 9.2. Absolute continuity of images of measures 282 9.3. Smoothness of induced measures 288 9.4. Infinite dimensional oscillatory integrals 297 9.5. Surface measures 299 9.6. Convergence of nonlinear images of measures 307 9.7. Supports of induced measures 319 9.8. Comments and exercises 323 Chapter 10. Infinite dimensional transformations 329 10.1. Linear transformations of Gaussian measures 329 10.2. Nonlinear transformations of Gaussian measures 334 10.3. Transformations of smooth measures 338 10.4. Absolutely continuous flows 340 10.5. Negligible sets 342 10.6. Infinite dimensional Rademacher’s theorem 350 10.7. Triangular and optimal transformations 358 10.8. Comments and exercises 365 Chapter 11. Measures on manifolds 369 11.1. Measurable manifolds and Malliavin’s method 370 11.2. Differentiable families of measures 379 11.3. Current and loop groups 390 11.4. Poisson spaces 393 11.5. Diffeomorphism groups 394 11.6. Comments and exercises 398 Chapter 12. Applications 401 12.1. A probabilistic approach to hypoellipticity 401 12.2. Equations for measures 407 12.3. Logarithmic gradients and symmetric diffusions 414 12.4. Dirichlet forms and differentiable measures 416 12.5. The uniqueness problem for invariant measures 420 12.6. Existence of Gibbs measures 422 12.7. Comments and exercises 424 References 427 Subject Index 483
Preface
Among the most notable events in nonlinear functional analysis for the last three decades one should mention the development of the theory of differentiable measures and the Malliavin calculus. These two closely related theories can be regarded at the same time as infinite dimensional analogues of such classical fields as geometric measure theory, the theory of Sobolev spaces, and the theory of generalized functions. The theory of differentiable measures was suggested by S.V. Fomin in his address at the International Congress of Mathematicians in Moscow in 1966 as a candidate for an infinite dimensional substitute of the Sobolev– Schwartz theory of distributions (see [440]–[443], [82], [83]). It was Fomin who realized that in the infinite dimensional case instead of a pair of spaces (test functions – generalized functions) it is natural to consider four spaces: a certain space of functions S, its dual S0, a certain space of measures M, and its dual M 0. In the finite dimensional case, M is identified with S by means of Lebesgue measure which enables us to represent measures via their densities. The absence of translationally invariant nonzero measures destroys this identification in the infinite dimensional case. Fomin found a convenient definition of a smooth measure on an infinite dimensional space corresponding to a measure with a smooth density on IRn. By means of this definition one introduces the space M of smooth measures. The Fourier transform then acts between S and M and between S0 and M 0. In this way a theory of pseudo-differential operators can be developed: the initial objective of Fomin was a theory of infinite dimensional partial differential equations. However, as it often happens with fruitful ideas, the theory of differentiable measures overgrew the initial framework. This theory has be- come an efficient tool in a wide variety of the most diverse applications such as stochastic analysis, quantum field theory, and nonlinear analysis. At present it is a rapidly developing field abundant with many challeng- ing problems of great importance for better understanding of the nature of infinite dimensional phenomena. It is worth mentioning that before the pioneering works of Fomin re- lated ideas appeared in several papers by T. Pitcher on the distributions of random processes in functional spaces. T. Pitcher investigated an even more general situation, namely he studied the differentiability of a family
ix x Preface
−1 of measures µt = µ◦Tt generated by a family of transformations Tt of a fixed measure µ. Fomin’s differentiability corresponds to the case where the measures µt are the shifts µth of a measure µ on a linear space. However, the theory constructed for this more special case is much richer. Let us also mention similar ideas of L. Gross [322] developed in the analysis on Wiener space. In the mid 1970s P. Malliavin [751] suggested an elegant method for proving the smoothness of the transition probabilities of finite dimensional diffusions. The essence of the method is to consider the transition proba- bilities Pt as images of the Wiener measure under some nonlinear transfor- mations (generated by stochastic differential equations) and then apply an integration by parts formula on the Wiener space leading to certain estimates ∞ of the generalized derivatives of Pt which ensure the membership in C . As an application, a probabilistic proof of H¨ormander’s theorem on hypoellip- tic second order operators was given. Malliavin’s method (now known as the Malliavin calculus or the stochastic variational calculus) attracted con- siderable attention and was refined and developed by many authors from different countries, including P. Malliavin himself. It was soon realized that the Malliavin calculus is a powerful tool for proving the differentiability of the densities of measures induced by finite dimensional nonlinear mappings of measures on infinite dimensional spaces (under suitable assumptions of regularity of the mappings and measures). Moreover, the ideas of the Malli- avin calculus have proved to be very efficient in many applications such as stochastic differential equations, filtering of stochastic processes, analysis on infinite dimensional manifolds, asymptotic behavior of heat kernels on fi- nite dimensional Riemannian manifolds (including applications to the index theorems), and financial mathematics. Although the theory of differentiable measures and the Malliavin calcu- lus have a lot in common and complement each other in many respects, their deep connection was not immediately realized and explored. The results of this exploration have turned out to be exciting and promising. Let us consider several examples illuminating, on the one hand, the ba- sic ideas of the theory of differentiable measures and the Malliavin calculus, and, on the other hand, explaining why the analysis on the Wiener space (or the Gaussian analysis) is not sufficient for dealing with smooth measures in infinite dimensions. First of all, which measures, say, on a separable Hilbert space X, should be regarded as infinite dimensional analogues of absolutely continuous measures on IRn? It is well-known that there is no canonical infi- nite dimensional substitute for Lebesgue measure. For example, no nonzero locally finite Borel measure µ on l2 is invariant or quasi-invariant under translations, that is, if µ ∼ µ(· + h) for every h, then µ is identically zero. Similarly, if µ is not the Dirac measure at the origin, it cannot be invariant under all orthogonal transformations of l2. In the finite dimensional case, in- stead of referring to Lebesgue measure one can fix a nondegenerate Gaussian Preface xi measure γ. Then such properties as the existence of densities or regularity of densities can be described via densities with respect to γ. For example, it is possible to introduce Sobolev classes over γ as completions of the class of smooth compactly supported functions with respect to certain natural Sobolev norms. The Malliavin calculus pursues this possibility and enables us to study measures whose Radon–Nikodym densities with respect to γ are elements of such Sobolev classes. However, in the infinite dimensional case, there is a continuum of mutually singular nondegenerate Gaussian measures (e.g., all measures γt(A) = γ(tA), t > 0, are mutually singular). Therefore, by fixing one of them, one restricts significantly the class of measures which can be investigated. Moreover, even if we do not fix a particular Gaussian measure, but consider measures for which there exist dominating Gaussian measures, we exclude from our considerations wide classes of smooth mea- sures arising in applications. Let us consider an example of such a measure with an especially simple structure.
Example 1. Let µ be the countable product of measures mn on the n n real line given by densities pn(t) = 2 p(2 t), where p is a smooth probabil- ity density with bounded support and finite Fisher’s information, i.e., the integral of (p0)2p−1 is finite. It is clear that µ is a probability measure on l2. The measure µ is mutually singular with every Gaussian measure (which will be proven in §5.3). Certainly, this measure µ can be regarded and has the same property on IR∞ (the countable power of the real line). Note that µ is an invariant probability for the diffusion process ξt governed by the following stochastic differential equation (on the space IR∞): 1 dξ = dW + β(ξ )dt, (1) t t 2 t ∞ n where the Wiener process Wt in IR is a sequence (wt ) of independent real Wiener processes, and the vector field β is given by