Random $C^*$-subalgebras

Abstract

Given a discrete countable group $Γ$, we introduce the concept of random subalgebras of the reduced group $C^*$-algebra $C_r^*(Γ)$. This extends the existing theory of invariant random subgroups and the one of invariant random subalgebras of the group von Neumann algebra ${\rm L}(Γ)$. As applications, we provide a characterisation of $C^*$-simplicity, a condition for amenability of the group and show the existence and uniqueness of a maximal nuclear $Γ$-invariant unital $C^*$-subalgebra of $C^*_r(Γ)$. Moreover, we give a topological characterisation of exactness of any separable unital $C^*$-algebra. Finally, we study exotic random subalgebras of $C^*_r(Γ)$ and super-exotic random subalgebras of ${\rm L}(Γ)$, that is, random subalgebras which do not arise purely from random subgroups of $Γ$ and not from random subalgebras of $C^*_r(Γ)$, respectively.