Propriétés des applications quadratiques d'un espace de Hilbert dans un espace vectoriel. Applications à la théorie spectrale des opérateurs normaux et à l'image numérique d'un opérateur
Nous montrons que admet une norme équivalente ce qui répond négativement à une question de Dowling, Hu et Smith. Puis nous obtenons une propriété de stabilité des opérateurs de Radon-Nikodym analytique. Motivés par l’identification entre et où est un espace de Banach, pour un groupe abélien compact métrisable , son dual , et , nous prouvons que, si l’espace a la propriété , alors il coincïde avec
The notion of proximal normal structure is introduced and used to study mappings that are "relatively nonexpansive" in the sense that they are defined on the union of two subsets A and B of a Banach space X and satisfy ∥ Tx-Ty∥ ≤ ∥ x-y∥ for all x ∈ A, y ∈ B. It is shown that if A and B are weakly compact and convex, and if the pair (A,B) has proximal normal structure, then a relatively nonexpansive mapping T: A ∪ B → A ∪ B satisfying (i) T(A) ⊆ B and T(B) ⊆ A, has a proximal point in the sense that...
In this paper it is shown that for a Brandt semigroup over a group with an arbitrary index set , if is amenable, then the Banach semigroup algebra is pseudo-amenable.
This paper introduces the following definition: a closed subspace Z of a Banach space E is pseudocomplemented in E if for every linear continuous operator u from Z to Z there is a linear continuous extension ū of u from E to E. For instance, every subspace complemented in E is pseudocomplemented in E. First, the pseudocomplemented hilbertian subspaces of are characterized and, in with p in [1, + ∞[, classes of closed subspaces in which the notions of complementation and pseudocomplementation...