Some Sharp Norm Estimates in the Subspace Perturbation Problem
نویسندگان
چکیده
We discuss the spectral subspace perturbation problem for a selfadjoint operator. Assuming that the convex hull of a part of its spectrum does not intersect the remainder of the spectrum, we establish an a priori sharp bound on variation of the corresponding spectral subspace under off-diagonal perturbations. This bound represents a new, a priori, tanΘ Theorem. We also extend the Davis–Kahan tan2Θ Theorem to the case of some unbounded perturbations.
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