Characterisations of Fourier and Fourier--Stieltjes algebras on locally compact groups

TL;DR

This paper characterizes Fourier and Fourier--Stieltjes algebras on locally compact groups using the framework of F-algebras, providing new insights into their structure and related algebraic properties.

Contribution

It offers novel characterizations of Fourier algebras as commutative semisimple F-algebras with specific properties, extending previous work by Rieffel and Walter.

Findings

Characterization of Fourier algebras as commutative semisimple F-algebras

Criteria for when a subalgebra of A(G) equals A(G)

New results on representations of discrete groups

Abstract

Motivated by the beautiful work of M. A. Rieffel (1965) and of M. E. Walter (1974), we obtain characterisations of the Fourier algebra $A(G)$ of a locally compact group $G$ in terms of the class of $F$-algebras (i.e. a Banach algebra $A$ such that its dual $A'$ is a $W^*$-algebra whose identity is multiplicative on $A$). For example, we show that the Fourier algebras are precisely those commutative semisimple $F$-algebras that are Tauberian, contain a nonzero real element, and possess a dual semigroup that acts transitively on their spectrums. Our characterisations fall into three flavours, where the first one will be the basis of the other two. The first flavour also implies a simple characterisation of when the predual of a Hopf-von Neumann algebra is the Fourier algebra of a locally compact group. We also obtain similar characterisations of the Fourier--Stieltjes algebras of $G$. En route, we prove some new results on the problem of when a subalgebra of $A(G)$ is the whole algebra and on representations of discrete groups.