Variation of Discrete Spectra for Non-Selfadjoint Perturbations of Selfadjoint Operators

Let B = A + K where A is a bounded selfadjoint operator and K is an element of the von Neumann–Schatten ideal ${\mathcal{S}_{p}}$ with p > 1. Let {λ _n } denote an enumeration of the discrete spectrum of B. We show that ${\sum_n {\rm dist}(\lambda_n, \sigma(A))^p}$ is bounded from above by a constant multiple of ${\|K\|_{p}^p}$ . We also derive a unitary analog of this estimate and apply it to obtain new estimates on zero-sets of Cauchy transforms.