Commutator estimates in $W^*$-factors

Let $\mathcal{M}$ be a $W^*$-factor and let $S\left( \mathcal{M} \right)$ be the space of all measurable operators affiliated with $\mathcal{M}$. It is shown that for any self-adjoint element $a\in S(\mathcal{M})$ there exists a scalar $λ_0\in\mathbb{R}$, such that for all $\varepsilon > 0$, there exists a unitary element $u_\varepsilon$ from $\mathcal{M}$, satisfying $|[a,u_\varepsilon]| \geq (1-\varepsilon)|a-λ_0\mathbf{1}|$. A corollary of this result is that for any derivation $δ$ on $\mathcal{M}$ with the range in an ideal $I\subseteq\mathcal{M}$, the derivation $δ$ is inner, that is $δ(\cdot)=δ_a(\cdot)=[a,\cdot]$, and $a\in I$. Similar results are also obtained for inner derivations on $S(\mathcal{M})$.

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