Convergence of noncommutative spherical averages for actions of free groups

TL;DR

This paper extends ergodic theorems for spherical averages in noncommutative spaces, broadening the scope from $L ext{log}L$ to more general Orlicz spaces, and establishes related convergence and Rota theorems.

Contribution

It generalizes the Bufetov pointwise ergodic theorem and Rota theorem to noncommutative Orlicz spaces, advancing the understanding of ergodic behavior in these settings.

Findings

Extended ergodic theorem to noncommutative Orlicz spaces

Proved Rota theorem in broader noncommutative spaces

Analyzed convergence of spherical averages for free group actions

Abstract

In this article, we extend the Bufetov pointwise ergodic theorem for spherical averages of even radius for free group actions on noncommutative $L\log L$-space. Indeed, we extend it to more general Orlicz space $L^\Phi(M, \tau)$ (noncommutative/classical), where $M$ is the semifinite von Neumann algebra with faithful normal semifinite trace $\tau$ and $\Phi: [0, \infty ) $ is a Orlicz function such that $ [0, \infty ) \ni t \rightarrow \left({\Phi(t)}\right)^{ \frac{1}{p}}$ is convex for some $p >1$. To establish this convergence we follow similar approach as Bufetov and Anantharaman-Delaroche. Thus, additionally we obtain Rota theorem on the same noncommutative Orlicz space by extending the earlier work of Anantharaman-Delaroche. Anantharaman-Delaroche proved Rota theorem for noncommutative $L^p$-spaces for $p >1$, and mentioned as ``interesting open problem'' to extend it to noncommutative $L\log L$-space as classical case. In the end we also look at the convergence of averages of spherical averages associated to free group and free semigroup actions on noncommutative spaces.