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In this paper we answer a question raised in [12] in the
affirmative, namely that the essential minimum modulus of an element in a von
Neumann algebra, relative to any norm closed two-sided ideal, is equal to the
minimum modulus of the element perturbed by an element from the ideal. As a
corollary of this result, we extend some basic perturbation results on semi-Fredholm
elements to a von Neumann algebra setting. We can characterize the semi-Fredholm
elements in terms of the points of continuity of the essential minimum modulus
function.
Mathematics Subject Classification (2000): 46L
Keywords: algebra, selfadjoint operator algebras, Fredholm theory,
von Neumann algebra, minimum modulus, semi-Fredholm



Quaestiones Mathematicaes 24 (4) 2001, 493–500