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Advances in Operator Theory, 2023, 8(3), 41
Perturbation classes problem
Semi-Fredholm operator
Strictly singular operator
Let SS and SC be the strictly singular and the strictly cosingular operators acting between Banach spaces, and let PΦ+ and PΦ+ be the perturbation classes for the upper and the lower semi-Fredholm operators. We study two classes of operators ΦS and ΦC that satisfy SS ⊂ ΦS ⊂ PΦ+ and SC ⊂ ΦC ⊂ PΦ_. We give some conditions under which these inclusions become equalities, from whichwe derive some positive solutions to the perturbation classes problem for semi-Fredholm operators.