Fredholm perturbations and seminorms related to upper semi-Fredholm perturbations
Fredholm perturbations and seminorms related to upper semi-Fredholm perturbations
In this paper, we investigate the stability in the set of Fredholm perturbations under composition with bounded operators. Moreover, we introduce the concept of a measure of non-Fredholm perturbations, which allows us to give a general approach to the question of obtaining perturbation theorems for semi-Fredholm operators. Finally, we prove …