Properties expressible in small fragments of the theory of the hyperfinite II_1 factor

TL;DR

This paper demonstrates that II$_1$ factors sharing the same 4-quantifier theory as the hyperfinite II$_1$ factor satisfy key embedding properties, extending previous results under weaker assumptions.

Contribution

It shows that sharing the same 4-quantifier theory with the hyperfinite II$_1$ factor implies important structural properties, improving prior results that required elementary equivalence.

Findings

Factors with same 4-quantifier theory as $ $ satisfy FCEP and Brown property.

Weakens previous assumptions from elementary equivalence to 4-quantifier theory.

Extends results on amalgamated free products over property (T) factors.

Abstract

We show that any II$_1$ factor that has the same 4-quantifier theory as the hyperfinite II$_1$ factor $\mathcal{R}$ satisfies the conclusion of the Popa Factorial Commutant Embedding Problem (FCEP) and has the Brown property. These results improve recent results proving the same conclusions under the stronger assumption that the factor is actually elementarily equivalent to $\mathcal{R}$. In the same spirit, we improve a recent result of the first-named author, who showed that if (1) the amalgamated free product of embeddable factors over a property (T) base is once again embeddable, and (2) $\mathcal{R}$ is an infinitely generic embeddable factor, then the FCEP is true of all property (T) factors. In this paper, it is shown that item (2) can be weakened to assume that $\mathcal{R}$ has the same 3-quantifier theory as an infinitely generic embeddable factor.