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 S_p with p>1. Let {\lambda_n} denote an enumeration of the discrete spectrum of B. We show that $\sum_n \dist(\lambda_n, \sigma(A))^p$ 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. 
Marcel Hansmann
math.SP math.CV