Quasi-Fredholm spectrum for operator matrices
Ouali El, El Ech-Cherif, Mohammed Karmouni · Filomat · 2022
For A ? L(X), B ? L(Y) and C ? L(Y,X) we denote by MC the operator matrix defined on X ? Y by MC = (A C 0 B). In this paper, we prove that ?qF(A) ? ?qF(B) ? [ C?L(Y,X) ?qF(MC) ? ?p(B) ? ?p(A?), where ?qF(.) (resp. ?p(.)) denotes the quasi-Fredholm spectrum (resp. the point spectrum). Furthermore, we consider some sufficient conditions for MC to be quasi-Fredholm and sufficient conditions to have ?qF(A) ? ?qF(B) = ? C?L(Y,X) ?qF(MC).