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

Abstract

Let $\mathcal{M}$ be a type ${\rm II_1}$ factor and let $τ$ 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 $E_jXE_j=τ(X) E_j$ for all $j=1,\cdots,N$. Equivalently, there is a unitary operator $U \in \mathcal{M}$ with $U^N=I$ and $\frac{1}{N}\sum_{j=0}^{N-1}{U^*}^jXU^j=τ(X)I.$ 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 $τ(A)\geq τ(R_A)$, 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 $τ(X)=0$, 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 $\frac{1}{k}\sum_{i=1}^kU_i^{-1}X_jU_i=τ(X_j)I,\quad \forall 1\leq j\leq n$. This result is a stronger version of Dixmier's averaging theorem for type ${\rm II}_1$ factors.