L. Elsner et S. Friedland, VARIATION OF THE DISCRETE EIGENVALUES OF NORMAL OPERATORS, Proceedings of the American Mathematical Society, 123(8), 1995, pp. 2511-2517
The Hoffman-Wielandt inequality, which gives a bound for the distance
between the spectra of two normal matrices, is generalized to normal o
perators A, B on a separable Hilbert space, such that A - B is Hilbert
-Schmidt.