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arXiv · 1709.01840

Hilbert space operators with compatible off-diagonal corners

L. Livshits·G. MacDonald·L. W. Marcoux·H. Radjavi
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Abstract

Given a complex, separable Hilbert space $\mathcal{H}$, we characterize those operators for which $\| P T (I-P) \| = \| (I-P) T P \|$ for all orthogonal projections $P$ on $\mathcal{H}$. When $\mathcal{H}$ is finite-dimensional, we also obtain a complete characterization of those operators for which $\mathrm{rank}\, (I-P) T P = \mathrm{rank}\, P T (I-P)$ for all orthogonal projections $P$. When $\mathcal{H}$ is infinite-dimensional, we show that any operator with the latter property is normal, and its spectrum is contained in either a line or a circle in the complex plane.

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L. Livshits, G. MacDonald, L. W. Marcoux, H. Radjavi. 2017-09-06. Hilbert space operators with compatible off-diagonal corners. https://arxiv.org/abs/1709.01840

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