Fredholm, Semi-Fredholm Perturbations, and Essential Spectra

Linear Operators. Part I: General Theory.

Paul Civin, Nelson Dunford, J. T. Schwartz · American Mathematical Monthly · 1960 · 2.8K citations

Natural Sciences, Mathematics, Algebra +11

Perturbation theory for nullity, deficiency and other quantities of linear operators

Tosio Kato · Journal d Analyse Mathématique · 1958 · 524 citations

Natural Sciences, Mathematics, Differential Equations +13

Two-Sided Ideals and Congruences in the Ring of Bounded Operators in Hilbert Space

J. W. Calkin · Annals of Mathematics · 1941 · 319 citations

Natural Sciences, Mathematics, Algebra +14

Unbounded Linear Operators.

U. D'Ambrosio, Seymour Goldberg · American Mathematical Monthly · 1968 · 292 citations

Semigroups of operators and measures of noncompactness

Arnold Lebow, Martin Schechter · Journal of Functional Analysis · 1971 · 193 citations

Natural Sciences, Mathematics, Set Theory +5