Model Theory of General von Neumann Algebras I: Generalized Ocneanu Ultraproducts

TL;DR

This paper develops a new axiomatization and metric ultraproduct framework for general von Neumann algebras, extending previous results and proving the undecidability of certain theories, including that of the hyperfinite II$_ _ $ factor.

Contribution

It introduces a generalized language and axiomatization for full left Hilbert algebras and explores their ultraproducts, extending prior work to a broader setting.

Findings

Generalizes ultraproduct results to non-$ $-finite von Neumann algebras.

Proves the undecidability of the theory of the hyperfinite II$_ _ $ factor.

Provides operator-algebraic characterizations of ultraproducts resembling known frameworks.

Abstract

This paper collates, presents, and expands upon technology and results obtained as part of the author's PhD thesis. We generalize work done in the $\sigma$-finite setting by the author, Goldbring, Hart, and Sinclair by producing a language and axiomatization of full left Hilbert algebras. To improve the accessibility of using this axiomatization, we examine the metric structure ultraproduct associated to this axiomatization. In doing so, we generalize results of Ando-Haagerup and Masuda-Tomatsu. This examination leads us to multiple operator-algebraic characterizations of the ultraproduct which closely resemble known characterizations of the Ocneanu ultraproduct. One of these is closely related to the notion of continuous elements of an ultraproduct with respect to an action. In the spirit of results of the author, Goldbring, and Hart, we prove various undecidable universal theory results for von Neumann algebras with unbounded weights. Notably, we prove that the hyperfinite II$_\infty$ factor together with its canonical tracial weight has an undecidable theory. This result was expected by experts, but could not be made precise until now. We also study the strengthenings of the negative solution to CEP that this result implies.