Some Fredholm Theory Results Around Relative Demicompactness Concept
Commun. Korean Math. Soc.
Published online December 11, 2020
Wajdi Chaker, Aref Jeribi, and Bilel Krichen
university of Sfax
Abstract : In this paper, we provide a characterization of upper semi-Fredholm
operators via the relative demicompactness concept. The obtained results are
used to investigate the stability of various essential spectra of closed linear
operators under perturbations belonging to classes involving demicompact, as
well as, relative demicompact operators.
Keywords : Fredholm Theory, demicompact operator, essential spectrum.
MSC numbers : 47A10, 47A55, 47A53.
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