Ergodic theorems for coset spaces

Michael Bjorklund, Alexander Fish

Abstract

We study in this paper the validity of the mean ergodic theorem along left Følner sequences in a countable amenable group $G$. Although the weak ergodic theorem always holds along any left Følner sequence in $G$, we provide examples where the mean ergodic theorem fails in quite dramatic ways. On the other hand, when $G$ does not admit any ICC quotients, e.g. if $G$ is virtually nilpotent, we prove that the mean ergodic theorem does indeed hold along any left Følner sequence. In the case when a unitary representation of any countable amenable group $G$ is induced from a "sufficiently thin" subgroup, we prove that the mean ergodic theorem holds along any left Følner sequence in $G$ for this representation. Furthermore, we show that every countable (infinite) amenable group $L$ embeds into a countable group $G$ which admits a unitary representation with the property that for any left Følner sequence $(F_n)$ in $L$, there exists a sequence $(s_n)$ in $G$ such that the mean (but not the weak) ergodic theorem fails for this representation along the sequence $(F_n{s}_n)$. Finally, we provide examples of countable (not necessarily amenable) groups $G$ with proper, infinite-index subgroups $H$, so that the pointwise ergodic theorem holds for averages along any strictly increasing and nested sequence of finite subsets of the coset $G/H$.