Some Fredholm theory results around relative demicompactness concept
Commun. Korean Math. Soc. 2021 Vol. 36, No. 2, 313-325
Published online December 11, 2020
Printed April 30, 2021
Wajdi Chaker, Aref Jeribi, Bilel Krichen
Higher Institute of Comercial Studies of Sfax; Faculty of Sciences of Sfax; Faculty of Sciences 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 : Primary 47A10; Secondary 47A55, 47A53
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