On the singular abelian rank of ultraproduct II$_1$ factors

Abstract

We prove that, under the continuum hypothesis $\frak c=\aleph_1$, any ultraproduct II$_1$ factor $M= \prod_{\omega} M_n$ of separable finite factors $M_n$ contains more than $\frak c$ many mutually disjoint singular MASAs, in other words the {\it singular abelian rank of} $M$, $\text{\rm r}(M)$, is larger than $ \frak c$. Moreover, if the strong continuum hypothesis $2^{\frak c}=\aleph_2$ is assumed, then ${\text{\rm r}}(M) = 2^{\frak c}$. More generally, these results hold true for any II$_1$ factor $M$ with unitary group of cardinality $\frak c$ that satisfies the bicommutant condition $(A_0'\cap M)'\cap M=M$, for all $A_0\subset M$ separable abelian.