A sufficient condition for the stability of the range of a positive operator in a Hilbert space is given. As a consequence, we get a class of additive perturbations which preserve regularity of the critical point 0 of a positive operator in a Krein space.
Proceedings of the American Mathematical Society
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First published in "Proceedings of the American Mathematical Society" in 1997, published by the American Mathematical Society.
Ćurgus, Branko and Najman, Branko, "Preservation of the Range Under Perturbations of an Operator" (1997). Mathematics. 11.