Commun. Korean Math. Soc. 2021; 36(2): 313-325
Online first article December 11, 2020 Printed April 30, 2021
https://doi.org/10.4134/CKMS.c200215
Copyright © The Korean Mathematical Society.
Wajdi Chaker, Aref Jeribi, Bilel Krichen
Higher Institute of Comercial Studies of Sfax; Faculty of Sciences of Sfax; Faculty of Sciences of Sfax
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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