Some Fredholm theory results around relative demicompactness concept Commun. Korean Math. Soc. 2021 Vol. 36, No. 2, 313-325 https://doi.org/10.4134/CKMS.c200215 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 Downloads: Full-text PDF   Full-text HTML