Kadison-Kastler stable factors

TL;DR

This paper proves that certain nonamenable von Neumann algebras, including those from free ergodic actions of SL_n(Z), are stable under small perturbations, confirming a long-standing conjecture for these classes.

Contribution

It establishes the Kadison-Kastler stability for a new class of nonamenable von Neumann algebras, including those arising from free ergodic actions of SL_n(Z).

Findings

First nonamenable von Neumann algebras satisfying the conjecture.

Stability results for crossed products with vanishing second bounded cohomology.

Applicable to free groups and similar structures.

Abstract

A conjecture of Kadison and Kastler from 1972 asks whether sufficiently close operator algebras in a natural uniform sense must be small unitary perturbations of one another. For $n\geq 3$ and a free ergodic probability measure preserving action of $SL_n(\mathbb Z)$ on a standard nonatomic probability space $(X,\mu)$, write $M=((L^\infty(X,\mu)\rtimes SL_n(\mathbb Z))\,\overline{\otimes}\, R$, where $R$ is the hyperfinite II$_1$ factor. We show that whenever $M$ is represented as a von Neumann algebra on some Hilbert space $\mathcal H$ and $N\subseteq\mathcal B(\mathcal H)$ is sufficiently close to $M$, then there is a unitary $u$ on $\mathcal H$ close to the identity operator with $uMu^*=N$. This provides the first nonamenable class of von Neumann algebras satisfying Kadison and Kastler's conjecture. We also obtain stability results for crossed products $L^\infty(X,\mu)\rtimes\Gamma$ whenever the comparison map from the bounded to usual group cohomology vanishes in degree 2 for the module $L^2(X,\mu)$. In this case, any von Neumann algebra sufficiently close to such a crossed product is necessarily isomorphic to it. In particular, this result applies when $\Gamma$ is a free group.