The noncommutative factor theorem for lattices in product groups

TL;DR

This paper establishes a noncommutative factor theorem for lattices in product groups, extending classical results to the setting of von Neumann algebras and providing a comprehensive classification of intermediate subalgebras for certain group actions.

Contribution

It proves a noncommutative Bader-Shalom factor theorem and derives a Margulis-type factor theorem for irreducible lattices in higher rank semisimple groups, describing all intermediate von Neumann subalgebras.

Findings

Proved a noncommutative Bader-Shalom factor theorem.

Obtained a noncommutative Margulis factor theorem for higher rank lattices.

Classified all intermediate von Neumann subalgebras between group von Neumann algebra and group measure space algebra.

Abstract

We prove a noncommutative Bader-Shalom factor theorem for lattices with dense projections in product groups. As an application of this result and our previous works, we obtain a noncommutative Margulis factor theorem for all irreducible lattices $\Gamma < G$ in higher rank semisimple algebraic groups. Namely, we give a complete description of all intermediate von Neumann subalgebras $\operatorname{L}(\Gamma) \subset M \subset \operatorname{L}(\Gamma \curvearrowright G/P)$ sitting between the group von Neumann algebra and the group measure space von Neumann algebra associated with the action on the Furstenberg-Poisson boundary.