The functional volume product under heat flow

Abstract

We prove that the functional volume product for even functions is increasing along the Fokker–Planck heat flow. This in particular yields a new proof of the functional Blaschke–Santaló inequality by K. Ball and Artstein-Avidan–Klartag–Milman in the even case. This result is a consequence of a new understanding of the regularizing property of the Ornstein–Uhlenbeck semigroup. That is, we establish an improvement of Borell’s reverse hypercontractivity inequality for even functions and identify the sharp range of the admissible exponents. As another consequence of successfully identifying the sharp range for the inequality, we derive a sharp $L^p$-$L^q$ inequality for the Laplace transform for even functions. The best constant of the inequality is attained by centered Gaussians, which provides an analogous result to Beckner’s sharp Hausdorff–Young inequality. Our technical novelty in the proof is the use of the Brascamp–Lieb inequality for log-concave measures and Cramér–Rao’s inequality in this context.