B-Fredholm Spectra and Riesz Perturbations

M. Berkani, H. Zariouh

Let $T$ be a bounded linear Banach space operator and let $Q$ be a quasinilpotent one commuting with $T$. The main purpose of the paper is to show that we do not have $\sigma_{*}(T+Q)=\sigma_{*}(T)$ where $\sigma_{*}\in\{\sigma_{D},\sigma_{LD}\}$, contrary to what has been announced in the proof of Lemma 3.5 from M. Amouch, {Polaroid operators with SVEP and perturbations of property (gw)}, Mediterr. J. Math. {6} (2009), 461--470, where $\sigma_{D}(T)$ is the Drazin spectrum of $T$ and $\sigma_{LD}(T)$ its left Drazin spectrum. However, under the additional hypothesis $\operatorname{iso}\sigma_{ub}(T)=\emptyset$, the mentioned equality holds. Moreover, we study the preservation of various spectra originating from B-Fredholm theory under perturbations by Riesz operators.