On the instability of non-semi-Fredholm operators under compact perturbations
The stability of several natural sets of the non-semi-Fredholm operators in a separable Hilbert space under compact perturbation has been studied by \textit{R. Bouldin} [J. Math. Anal. Appl. 87, 632-638 (1982; Zbl 0504.47019)]. The aim of the present article is to study this problem in arbitrary Banach spaces. We also derive a curious characterization of separable Banach spaces.
scientific article; zbMATH DE number 3971548