On Mbekhta's conjecture and pseudo-Fredholm operators on Banach and Hilbert spaces

Abstract

We give negative answers to Mbekhta's conjecture \cite[p.~261]{Mbekhta} and to a related conjecture on pseudo-B-Fredholm operators \cite[p.~202]{AZO}. A sufficient condition for pseudo-Fredholmness on Banach spaces is established. It yields a characterization on Hilbert spaces in terms of the closedness of $\cc(T)$, $\cc(T)+\qn(T)$ and $\Ran(T)+\qn(T)$, as well as characterizations of pseudo-B-Fredholm and pseudo-B-Weyl operators on Banach spaces. A counterexample on a separable Banach space shows that the three closedness conditions do not characterize pseudo-Fredholmness in general. We also construct two operators having the same analytic core, quasinilpotent part, and kernels and ranges of all powers, although only one is pseudo-Fredholm. Moreover, a semi-B-Fredholm operator on a Banach space need not be a direct sum of a semi-Fredholm operator and a nilpotent operator, answering the question raised in \cite[p.~113]{AznayKato2023}. Finally, the three closedness conditions in the Hilbert-space characterization are shown to be independent.