Unitary representations of locally compact groups as metric structures

TL;DR

This paper demonstrates how the class of continuous unitary representations of a locally compact group can be modeled as an elementary class of metric structures within continuous logic, linking group representations with model-theoretic frameworks.

Contribution

It introduces a novel approach to represent continuous unitary representations as elementary classes in continuous logic, connecting group theory, operator algebras, and model theory.

Findings

Representations of $G$ as elementary classes in continuous logic.

Ultraproducts of representations relate to logical ultraproducts.

Characterization of property (T) via definability of fixed point sets.

Abstract

For a locally compact group $G$, we show that it is possible to present the class of continuous unitary representations of $G$ as an elementary class of metric structures, in the sense of continuous logic. More precisely, we show how non-degenerate $*$-representations of a general $*$-algebra $A$ (with some mild assumptions) can be viewed as an elementary class, in a many-sorted language, and use the correspondence between continuous unitary representations of $G$ and non-degenerate $*$-representations of $L^1(G)$. We relate the notion of ultraproduct of logical structures, under this presentation, with other notions of ultraproduct of representations appearing in the literature, and characterise property (T) for $G$ in terms of the definability of the sets of fixed points of $L^1$ functions on $G$.