Semi-Fredholm operators with finite ascent or descent and perturbations
Semi-Fredholm operators with finite ascent or descent and perturbations
In this note we prove that the collection of upper (lower) semi-Fredholm operators with finite ascent (descent) is closed under commuting operator perturbations that belong to the perturbation class associated with the set of upper (lower) semi-Fredholm operators. Then, as a corollary we get the main result of S. Grabiner …