Résumé : Let T be a bounded linear operator between two normed spaces Y and X. We characterize compactness of T in terms of differentiability of the Lipschitz functions defined on X with values in another normed space Z. Furthermore, using a similar technique we can also characterize finite rank operators interms of differentiability of a wider class of functions but still with Lipschitz flavour. As an application we obtain a Banach-Stone-like theorem. On the other hand, we give an extension of a result of Bourgain and Diestel related to limited operators and cosingularity.

This work was obtained jointly with Mohammed Bachir (Université Paris 1, SAMM) and Sebastian Tapia-Garcia (Université du Chili).