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Let $\mathcal{M}$ be a type ${\rm II_1}$ factor and let $\tau$ be thefaithful normal tracial state on $\mathcal{M}$. In this paper, we prove thatgiven an $X \in \mathcal{M}$, $X=X^*$, then there is a decomposition of theidentity into $N \in \mathbb{N}$ mutually orthogonal nonzero projections$E_j\in\mathcal{M}$, $I=\sum_{j=1}^NE_j$, such that $E_jXE_j=\tau(X) E_j$ forall $j=1,\cdots,N$. 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, weprove that a positive operator $A\in \mathcal{M}$ can be written as a finitesum 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 affirmativelyQuestion 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 answersaffirmatively 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 ${\rmII}_1$ factors.

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