Nonamenable groups whose reduced group C*-algebras are not pure

Abstract

We exhibit nonamenable groups whose reduced group C*-algebras are not pure. More precisely, if $Γ$ is any countably infinite discrete group, then the reduced group C*-algebra of the restricted wreath product $(\mathbb{Z}/2\mathbb{Z}) \wr Γ$ has an ideal-quotient isomorphic to $\mathcal{K}(\ell^2(Γ))$. It therefore fails to be nowhere scattered and, in particular, is not pure. Taking $Γ$ to be any nonamenable group, we thereby obtain a negative answer to a question of Thiel.