Successful development of the
theory of Fredholm and semi-Fredholm operators and a general recognition of
the importance of the subject provides an impetus for the study of various
generalizations. In this paper, a study is made of operators whose ranges and null
spaces are closed complemented subspaces. In particular, if T is such an operator, a
rather general sufficient condition is obtained to ensure that T − U is of the same
kind. This perturbation theorem includes, as special cases, previous results due to
Dieudonné, Saphar, and Crownover.