Convergence in Wasserstein Distance for Empirical Measures of Dirichlet Diffusion Processes on Manifolds

Abstract

Let $M$ be a $d$-dimensional connected compact Riemannian manifold with boundary $\partial M$, let $V\in C^2(M)$ such that $\mu({\rm d} x):={\rm e}^{V(x)}{\rm d} x$ is a probability measure, and let $X_t$ be the diffusion process generated by $L:=\Delta+\nabla V$ with $\tau:=\inf\{t\ge 0: X_t\in\partial M\}$. Consider the empirical measure $\mu_t:=\frac 1 t \int_0^t \delta_{X_s}{\rm d} s$ under the condition $t<\tau$ for the diffusion process. If $d\le 3$, then for any initial distribution not fully supported on $\partial M$, \begin{align*} &c\sum_{m=1}^\infty \frac{2}{(\lambda_m-\lambda_0)^2} \le \liminf_{t\to \infty} \inf_{T\ge t} \Big\{t {\mathbb E}\big[\mathbb W_2(\mu_t, \mu_0)^2\big|T<\tau\big]\Big\} \\ &\le \limsup_{t\to \infty} \sup_{T\ge t} \Big\{ t \mathbb E\big[\mathbb W_2(\mu_t, \mu_0)^2\big|T<\tau\big] \Big\}\le \sum_{m=1}^\infty \frac{2}{(\lambda_m-\lambda_0)^2}\end{align*} holds for some constant $c\in (0,1]$ with $c=1$ when $\partial M$ is convex, where $\mu_0:= \phi_0^2\mu$ for the first Dirichet eigenfunction $\phi_0$ of $L$, $\{\lambda_m\}_{m\ge 0}$ are the Dirichlet eigenvalues of $-L$ listed in the increasing order counting multiplicities, and the upper bound is finite if and only if $d\le 3$. When $d=4$, $\sup_{T\ge t} \mathbb E\big[\mathbb W_2(\mu_t, \mu_0)^2\big|T<\tau\big] $ decays in the order $t^{-1}\log t$, while for $d\ge 5$ it behaves like $t^{-\frac 2 {d-2}}$, as $t\to\infty$.