Geodesic
ON THE PERTURBATION CLASSES OF CONTINUOUS SEMI-FREDHOLM OPERATORS
Glasgow mathematical journal, Tome 45 (2003) no. 1, pp. 91-95

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DOI

We prove that the perturbation class of the upper semi-Fredholm operators from $X$ into $Y$ is the class of the strictly singular operators, whenever $X$ is separable and $Y$ contains a complemented copy of $C[0, 1]$. We also prove that the perturbation class of the lower semi-Fredholm operators from $X$ into $Y$ is the class of the strictly cosingular operators, whenever $X$ contains a complemented copy of $\ell_1$ and $Y$ is separable. We can remove the separability requirements by taking suitable spaces instead of $C[0, 1]$ or $\ell_1$.
DOI : 10.1017/S0017089502001027
Mots-clés : 47A53, 47A55 
AIENA, PIETRO; GONZÁLEZ, MANUEL; MARTINÓN, ANTONIO. ON THE PERTURBATION CLASSES OF CONTINUOUS SEMI-FREDHOLM OPERATORS. Glasgow mathematical journal, Tome 45 (2003) no. 1, pp. 91-95. doi: 10.1017/S0017089502001027
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