A note on the invariant subspace problem relative to a type ${\rm II}_1$ factor

TL;DR

This paper investigates the structure of operators in ultrapower algebras of type II_1 factors, demonstrating the existence of specific projections and approximations, and distinguishing certain ultrapower algebras from hyperfinite ones.

Contribution

It proves the existence of a family of projections for operators in ultrapower algebras and characterizes operators approximable by those with a specific projection structure.

Findings

Existence of a family of projections for operators in ultrapower algebras.

Operators in the algebra can be approximated by operators with a prescribed projection structure.

The ultrapower of the matrix algebras is not isomorphic to the ultrapower of the hyperfinite II_1 factor.

Abstract

Let $\M$ be a type ${\rm II}_1$ factor with a faithful normal tracial state $\tau$ and let $\M^\omega$ be the ultrapower algebra of $\M$. In this paper, we prove that for every operator $T\in \M^\omega$, there is a family of projections $\{P_t\}_{0\leq t\leq 1}$ in $\M^\omega$ such that $TP_t=P_tTP_t$, $P_s\leq P_t$ if $s\leq t$, and $\tau_\omega(P_t)=t$. Let $\mathfrak{M}=\{Z \in \M: \text{there is a family of projections} \{P_t\}_{0\leq t\leq 1} \text{in} \M \text{such that} ZP_t=P_tZP_t, P_s\leq P_t \text{if} s\leq t, \text{and} \tau(P_t)=t\}$. As an application we show that for every operator $T\in \M$ and $\epsilon>0$, there is an operator $S\in \mathfrak{M}$ such that $\|S\|\leq \|T\|$ and $\|S-T\|_2<\epsilon$. We also show that $\prod_n^\omega M_n(\cc)$ is not $\ast$-isomorphic to the ultrapower algebra of the hyperfinite type ${\rm II}_1$ factor.