On finite sums of projections and Dixmier's averaging theorem for type ${\rm II}_1$ factors

TL;DR

This paper proves a decomposition theorem for self-adjoint elements in type II_1 factors, leading to new results on sums of projections, nilpotent elements, and an enhanced version of Dixmier's averaging theorem.

Contribution

It introduces a finite sum decomposition of projections, characterizes when positive operators are sums of projections, and strengthens Dixmier's averaging theorem for type II_1 factors.

Findings

Decomposition of self-adjoint elements into projections with equal trace

Characterization of positive operators as finite sums of projections

Existence of unitaries averaging operators to scalar multiples of the identity

Abstract

Let $\mathcal{M}$ be a type ${\rm II_1}$ factor and let $\tau$ be the faithful normal tracial state on $\mathcal{M}$. In this paper, we prove that given an $X \in \mathcal{M}$, $X=X^*$, then there is a decomposition of the identity into $N \in \mathbb{N}$ mutually orthogonal nonzero projections $E_j\in\mathcal{M}$, $I=\sum_{j=1}^NE_j$, such that $E_jXE_j=\tau(X) E_j$ for all $j=1,\cdots,N$. Equivalently, there is a unitary operator $U \in \mathcal{M}$ with $U^N=I$ and $\frac{1}{N}\sum_{j=0}^{N-1}{U^*}^jXU^j=\tau(X)I.$ As the first application, we prove that a positive operator $A\in \mathcal{M}$ can be written as a finite sum of projections in $\mathcal{M}$ if and only if $\tau(A)\geq \tau(R_A)$, where $R_A$ is the range projection of $A$. This result answers affirmatively Question 6.7 of [9]. As the second application, we show that if $X\in \mathcal{M}$, $X=X^*$ and $\tau(X)=0$, then there exists a nilpotent element $Z \in \mathcal{M}$ such that $X$ is the real part of $Z$. This result answers affirmatively Question 1.1 of [4]. As the third application, we show that let $X_1,\cdots,X_n\in \mathcal{M}$. Then there exist unitary operators $U_1,\cdots,U_k\in\mathcal{M}$ such that $\frac{1}{k}\sum_{i=1}^kU_i^{-1}X_jU_i=\tau(X_j)I,\quad \forall 1\leq j\leq n$. This result is a stronger version of Dixmier's averaging theorem for type ${\rm II}_1$ factors.