Hilbert space operators with compatible off-diagonal corners (1709.01840v1)

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.