Uniform Central Limit Theorems for Multidimensional Diffusions

Abstract: It has recently been shown that there are substantial differences in the regularity behavior of the empirical process based on scalar diffusions as compared to the classical empirical process, due to the existence of diffusion local time. Besides establishing strong parallels to classical theory such as Ossiander's bracketing CLT and the general Gin\'e-Zinn CLT for uniformly bounded families of functions, we find increased regularity also for multivariate ergodic diffusions, assuming that the invariant measure is finite with Lebesgue density $\pi$. The effect is diminishing for growing dimension but always present. The fine differences to the classical iid setting are worked out using exponential inequalities for martingales and additive functionals of continuous Markov processes as well as the characterization of the sample path behavior of Gaussian processes by means of the generic chaining bound. To uncover the phenomenon, we study a smoothed version of the empirical diffusion process. It turns out that uniform weak convergence of the smoothed empirical diffusion process under necessary and sufficient conditions can take place with even exponentially small bandwidth in dimension $d=2$, and with strongly undersmoothing bandwidth choice for parameters $\beta > d/2$ in case $d\geq 3$, assuming that the coordinates of drift and diffusion coefficient belong to some H\"older ball with parameter $\beta$.