Positive definite functions as uniformly ergodic multipliers of the Fourier algebra

TL;DR

This paper characterizes when positive definite functions on a locally compact group induce uniformly ergodic multipliers on the Fourier algebra, linking ergodic properties to subgroup structure and spectral characteristics.

Contribution

It provides a complete classification of the ergodic behavior of these multipliers based on subgroup openness and spectral conditions, offering new insights into their structure.

Findings

Uniform mean ergodicity occurs iff $H_\phi$ is open and 1 is not an accumulation point of the spectrum.

Powers of $M_\phi$ converge in norm iff the operator is uniformly mean ergodic and $H_\phi$ equals the set where $|\phi|=1$.

Characterizes the ergodic properties of $M_\phi$ in terms of subgroup and spectral conditions.

Abstract

Let G be a locally compact group and let $\phi$ be a positive definite function on G with $\phi(e)=1$. This function defines a multiplication operator $M_\phi$ on the Fourier algebra $A(G)$ of $G$. The aim of this paper is to classify the ergodic properties of the operators $M_\phi$, focusing on several key factors, including the subgroup $H_\phi=\{x\in G\colon \phi(x)=1\}$, the spectrum of $M_\phi$, or how ``spread-out'' a power of $M_\phi$ can be. We show that the multiplication operator $M_\phi$ is uniformly mean ergodic if and only if $H_\phi$ is open and 1 is not an accumulation point of the spectrum of $M_\phi$. Equivalently, this happens when some power of $\phi$ is not far, in the multiplier norm, from a function supported on finitely many cosets of $H_\phi$. Additionally, we show that the powers of $M_\phi$ converge in norm if, and only if, the operator is uniformly mean ergodic and $H_\phi =\{x\in G\colon |\phi(x)|=1\}$.