Boundary maps, germs and quasi-regular representations

Abstract

We investigate the tracial and ideal structures of $C^*$-algebras of quasi-regular representations of stabilizers of boundary actions. Our main tool is the notion of boundary maps, namely $\Gamma$-equivariant unital completely positive maps from $\Gamma$-$C^*$-algebras to $C(\partial_F\Gamma)$, where $\partial_F\Gamma$ denotes the Furstenberg boundary of a group $\Gamma$. For a unitary representation $\pi$ coming from the groupoid of germs of a boundary action, we show that there is a unique boundary map on $C^*_\pi(\Gamma)$. Consequently, we completely describe the tracial structure of the $C^*$-algebras $C^*_\pi(\Gamma)$, and for any $\Gamma$-boundary $X$, we completely characterize the simplicity of the $C^*$-algebras generated by the quasi-regular representations $\lambda_{\Gamma/\Gamma_x}$ associated to stabilizer subgroups $\Gamma_x$ for any $x\in X$. As an application, we show that the $C^*$-algebra generated by the quasi-regular representation $\lambda_{T/F}$ associated to Thompson's groups $F\leq T$ does not admit traces and is simple.