A property (T) for C*-algebras

Abstract

We define a notion of Property (T) for an arbitrary $C^*$-algebra $A$ admitting a tracial state. We extend this to a notion of Property (T) for the pair $(A,B),$ where $B$ is a $C^*$-subalgebra of $A.$ Let $\Gamma$ be a discrete group and $C^*_r(\Gamma)$ its reduced algebra. We show that $C^*_r(\Gamma)$ has Property (T) if and only if the group $\Gamma$ has Property (T) . More generally, given a subgroup $\Lambda$ of $\Gamma$, the pair $(C^*_r(\Gamma),C^*_r(\Lambda))$ has Property (T) if and only if the pair of groups $(\Gamma, \Lambda)$ has Property (T).