Some sharp norm estimates in the subspace perturbation problem

Abstract

We discuss the spectral subspace perturbation problem for a self-adjoint operator. Assuming that the convex hull of a part of its spectrum does not intersect the remainder of the spectrum, we establish an \textit{a priori} sharp bound on variation of the corresponding spectral subspace under off-diagonal perturbations. This bound represents a new, \textit{a priori}, $\tanΘ$ Theorem. We also extend the Davis--Kahan $\tan 2Θ$ Theorem to the case of some unbounded perturbations.