On the structure of relatively biexact group von Neumann algebras

Abstract

Using computations in the bidual of $\mathbb{B}(L^2M)$ we develop a new technique at the von Neumann algebra level to upgrade relative proper proximality to full proper proximality. This is used to structurally classify subalgebras of $LΓ$ where $Γ$ is an infinite group that is biexact relative to a finite family of subgroups $\{Λ_i\}_{i\in I}$ such that each $Λ_i$ is almost malnormal in $Γ$. This generalizes the result of \cite{DKEP21} which classifies subalgebras of von Neumann algebras of biexact groups. By developing a combination with techniques from Popa's deformation-rigidity theory we obtain a new structural absorption theorem for free products and a generalized Kurosh type theorem in the setting of properly proximal von Neumann algebras.