Toplogical uniform descent, quasi-Fredholmness and operators originated from semi-B-Fredholm theory

Abstract

In this paper we study operators originated from semi-B-Fredholm theory and as a consequence we get some results regarding boundaries and connected hulls of the corresponding spectra. In particular, we prove that a bounded linear operator $T$ acting on a Banach space, having topological uniform descent, is a {\bf BR} operator if and only if $0$ is not an accumulation point of the associated spectrum $\sigma_{\bf R}(T)=\{\lambda\in\CC:T-\lambda I\notin {\bf R}\}$, where ${\bf R}$ denote any of the following classes: upper semi-Weyl operators, Weyl operators, upper semi-Fredholm operators, Fredholm operators, operators with finite (essential) descent and ${\bf BR}$ the B-regularity associated to ${\bf R}$ as in \cite{P8}. Under the stronger hypothesis of quasi-Fredholmness of $T,$ we obtain a similar characterization for $T$ being a {$\bf BR$} operator for much larger families of sets ${\bf R}.$