Paper detail

Uniqueness of trace and C*-simplicity beyond regular representation

A discrete group $Γ$ is C*-simple if the C*-algebra $C_λ^*(Γ)$ generated by the range of the left regular representation $λ$ on $\ell^2(Γ)$ is simple. In this case, $Γ$ acts faithfully on the Furstenberg boundary $\partial_FΓ$ and there is a unique trace on $C_λ^*(Γ)$. In this paper we study the unique trace property for the C*-algebra $C_π^*(Γ)$ generated by the range of an arbitrary unitary representation $π: Γ\to B(H_π) $ and relate it to the faithfulness of the action of $Γ$ on the Furstenberg-Hamana boundary $\mathcal B_π$. Similar relation is obtained between simplicity of $C_π^*(Γ)$ and (topological) freeness of the action of $Γ$ on $\mathcal B_π$. Along the way, we extend the Connes-Sullivan and Powers averaging properties for a unitary representation $π$ and relate them to simplicity and unique trace property of $C^*_π(Γ)$.