Malliavin's absolute continuity lemma

In mathematics — specifically, in measure theory — Malliavin's absolute continuity lemma is a result due to the French mathematician Paul Malliavin that plays a foundational rôle in the regularity (smoothness) theorems of the Malliavin calculus. Malliavin's lemma gives a sufficient condition for a finite Borel measure to be absolutely continuous with respect to Lebesgue measure.

Statement of the lemma

Let μ be a finite Borel measure on n-dimensional Euclidean space Rⁿ. Suppose that, for every x ∈ Rⁿ, there exists a constant C = C(x) such that

\left|\int _{\mathbf {R} ^{n}}\mathrm {D} \varphi (y)(x)\,\mathrm {d} \mu (y)\right|\leq C(x)\|\varphi \|_{\infty }

for every C^∞ function φ : Rⁿ → R with compact support. Then μ is absolutely continuous with respect to n-dimensional Lebesgue measure λⁿ on Rⁿ. In the above, Dφ(y) denotes the Fréchet derivative of φ at y and ||φ||_∞ denotes the supremum norm of φ.

References

Bell, Denis R. (2006). The Malliavin calculus. Mineola, NY: Dover Publications Inc. pp. x+113. ISBN 0-486-44994-7. MR 2250060 (See section 1.3)
Malliavin, Paul (1978). "Stochastic calculus of variations and hypoelliptic operators". Proceedings of the International Symposium on Stochastic Differential Equations (Res. Inst. Math. Sci., Kyoto Univ., Kyoto, 1976). New York: Wiley. pp. 195–263. MR 536013

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