Temporo-spatial differentiations for actions of locally compact groups

Abstract

In this paper, we extend the notion of temporo-spatial differentiation problems to the setting of actions of more general topological groups. The problem can be expressed as follows: Given an action $T$ of an amenable discrete group $G$ on a probability space $(X, \mu)$ by automorphisms, let $(F_k)_{k = 1}^\infty$ be a F{\o}lner sequence for $G$, and let $(C_k)_{k = 1}^\infty$ be a sequence of measurable subsets of $X$ with positive probability $\mu(C_k)$. What is the limiting behavior of the sequence $$\left( \frac{1}{\mu(C_k)} \int_{C_k} \frac{1}{|F_k|} \sum_{g \in F_k} f(T_g x) \mathrm{d} \mu(x) \right)_{k = 1}^\infty$$ for $f \in L^\infty(X, \mu)$? We provide some positive convergence results for temporo-spatial differentiations with respect to ergodic averages over F{\o}lner sequences, as well as with respect to ergodic averages over subsequences of the integers (e.g. polynomials), multiple ergodic averages, and weighted ergodic averages.