Cocompact amenable closed subgroups: weakly inequivalent representations in the left-regular representation

Abstract

We show that if $H\le G$ is a closed amenable and cocompact subgroup of a unimodular locally compact group, then the reduced group C*-algebra of $G$ is not simple. Equivalently, there are unitary representations of $G$ that are weakly contained in the left-regular representation, but not weakly equivalent to it. We discuss applications of this result and pose the problem to construct non-discrete topologically simple groups with a cocompact amenable closed subgroup but without a Gelfand pair.

arXiv commentv3: accepted for publication in Int. Math. Res. Not.; minor changes. v2: corrected typos and clarified proofs