In this note necessary and sufficient conditions for the regularity of the critical point infinity of a definitizable operator A are given. Using these criteria it is proved that the regularity of the critical point infinity is preserved under some additive perturbations as well as for some operators which are related to A. Applications to indefinite Sturm-Liouville problems are indicated.
Integral Equations and Operator Theory
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Ćurgus, Branko, "On the Regularity of the Critical Point Infinity of Definitizable Operators" (1985). Mathematics. Paper 75.