Non-classification of Cartan subalgebras for a class of von Neumann algebras

Abstract

We study the complexity of the classification problem for Cartan subalgebras in von Neumann algebras. We construct a large family of II${}_1$ factors whose Cartan subalgebras up to unitary conjugacy are not classifiable by countable structures, providing the first such examples. Additionally, we construct examples of II${}_1$ factors whose Cartan subalgebras up to conjugacy by an automorphism are not classifiable by countable structures. Finally, we show directly that the Cartan subalgebras of the hyperfinite II${}_1$ factor up to unitary conjugacy are not classifiable by countable structures, and deduce that the same holds for any McDuff II${}_1$ factor with at least one Cartan subalgebra.

arXiv commentv3: Final version. Theorem B generalized to all countable groups, exposition improved. To appear in Advances in Mathematics. v2: Corollary G on McDuff factors added