A Note on the Index of B-fredholm Operators

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From Corollary 3.5 in [Berkani, M; Sarih, M.; Studia Math. 148 (2001), 251– 257] we know that if S, T are commuting B-Fredholm operators acting on a Banach space X, then ST is a B-Fredholm operator. In this note we show that in general we do not have ind(ST ) = ind(S) + ind(T ), contrarily to what has been announced in Theorem 3.2 in [Berkani, M; Proc. Amer.Math. Soc. 130 (2002), 1717–1723]. However, if there exist U, V ∈ L(X) such that S, T , U , V are commuting and US + V T = I, then ind(ST ) = ind(S) + ind(T ), where ind stands for the index of a B-Fredholm operator. MSC 2000 : 47A53, 47A55

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تاریخ انتشار 2004