Second order differentiation formula on $RCD^*(K,N)$ spaces

TL;DR

This paper establishes a second order differentiation formula for functions along geodesics in $RCD^*(K,N)$ spaces, extending previous results to full generality and introducing new estimates for entropic interpolations.

Contribution

It proves the second order differentiation formula in $RCD^*(K,N)$ spaces without compactness assumptions, using novel estimates for entropic interpolations and approximation techniques.

Findings

Second order differentiation formula proved in $RCD^*(K,N)$ spaces.

New estimates for entropic interpolations, including density bounds and Lipschitz continuity.

Application to Hamilton-Jacobi equation via vanishing viscosity method.

Abstract

Aim of this paper is to prove the second order differentiation formula for $H^{2,2}$ functions along geodesics in $RCD^*(K,N)$ spaces with $N < \infty$. This formula is new even in the context of Alexandrov spaces, where second order differentiation is typically related to semiconvexity. We establish this result by showing that $W_2$-geodesics can be approximated up to second order, in a sense which we shall make precise, by entropic interpolation. In turn this is achieved by proving new, even in the smooth setting, estimates concerning entropic interpolations which we believe are interesting on their own. In particular we obtain: - equiboundedness of the densities along the entropic interpolations, - local equi-Lipschitz continuity of the Schr\"odinger potentials, - a uniform weighted $L^2$ control of the Hessian of such potentials. Finally, the techniques adopted in this paper can be used to show that in the $RCD$ setting the viscous solution of the Hamilton-Jacobi equation can be obtained via a vanishing viscosity method, in accordance with the smooth case. With respect to a previous version, where the space was assumed to be compact, in this paper the second order differentiation formula is proved in full generality.