A unitary analogue of Kato's theorem on variation of discrete spectra
They obtain an estimate of the distance between extended enumerations of the discrete spectra of two unitary operators whose difference is compact. The estimation is given in terms of the singular values of the difference. The result is analogous to a recent result of T. Kato for selfadjoint operators and so is the proof.
estimate of the distance between extended enumerations of the discrete spectra of two unitary operators whose difference is compact
scientific article; zbMATH DE number 4075870