Here, T(^p) denote^ the trace class of completely continuous operators in § and p(Hp) the resolvent set of Hp. The perturbation theory of absolutely continuous (abbr. a.c.) parts of Hp as well as the theory of wave and scattering operators has recently been studied independently by de Branges [2], Birman and Kreïn [ l ] , and Kato [3], In [ l ] and [3] the problem was considered from the viewpo...