Existential Inclusions of Bi-exact Groups are Conjugacy Representation Rigid

Abstract

If $Λ$ is a non-amenable bi-exact group and $Λ\hookrightarrow Γ$ is an existential embedding, then each of the intersections $Λ\cap g Λg^{-1}$ for $g$ a member of $Γ\backslash Λ$ is amenable. This in conjunction with work of Bekka and Kalantar demonstrates that in this situation, the weak equivalence class of the quasi-regular representation $λ_{Γ/Λ}$ determines $Λ$ up to conjugacy among the self-commensurating subgroups of $Γ$.