Convergence of noncommutative spherical averages for actions of free groups

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.