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Commutator inequalities associated with the polar decomposition
Authors:Fuad Kittaneh
Institution:Department of Mathematics, University of Jordan, Amman, Jordan
Abstract:Let $A=UP$ be a polar decomposition of an $n\times n$ complex matrix $A$. Then for every unitarily invariant norm $\vert\vert\vert\cdot\vert\vert\vert$, it is shown that

\begin{displaymath}\vert\vert\vert\, \vert UP-PU\vert^2\vert\vert\vert \le \vert... ...vert\le \Vert UP+PU\Vert\,\vert\vert\vert UP-PU\vert\vert\vert,\end{displaymath}

where $\Vert\cdot\Vert$ denotes the operator norm. This is a quantitative version of the well-known result that $A$ is normal if and only if $UP=PU$. Related inequalities involving self-commutators are also obtained.

Keywords:Commutator  polar decomposition  positive semidefinite matrix  unitarily invariant norm
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