Résumé: Let X be a a real normed linear space of dimension at least three, with unit sphere S-X. In this paper we prove that X is an inner product space if and only if every three point subset of S-X has a Chebyshev center in its convex hull. We also give other characterizations expressed in terms of centers of three point subsets of S-X only. We use in these characterizations Chebyshev centers
as well as Fermat centers and p-centers.
Mots-clés: Chebyshev centers; characterizations of inner product spaces
Résumé: We study some characterizations of inner product spaces given in the literature. Among other things, we give an example showing that one of the characterizations given in the classical book of Amir (1986) is not correct.
Résumé: La tesis se plantea como objetivo principal la caracterización de espacios prehilbert a través de propiedades de localización de centros de Chebishev, de centros de Fermat, de p-centros, o, con más generalidad, de gamma-centros (gamma norma monótona). El punto de partida es una caracterización en términos de centros de Chbyshev que presenta Amir en su libro (Characterizations of Inner Product Spaces, Birkhauser 1986). En la tesis, entre
otras cosas se prueba que la caracterización de Amir es falsa y se dan alterantivas para modificarla de modo que se obtengan caracterizaciones verdaderas. También se plantean y resuelven problemas de Aproximación simultánea en espacios de funciones integrables Bochener. Los pulmones de Aproximación simultánea se consideran tanto desde el punto de vista de Saidi, Hussein y Khalil, como del de Li y Watson.
Résumé: Let X be a Banach space, (Omega, Sigma, mu) a finite measure space, and L-1 (mu, X) the Banach space of X-valued Bochner mu-integrable functions defined on Omega endowed with its usual norm. Let us suppose that Sigma(0) is a sub-sigma-algebra of Sigma, and let mu(0) be the restriction of mu to Sigma(0). Given a natural number n, let N be a monotonous norm in R-n. It is shown that
if X is reflexive then L-1 (mu(0), X) is N-simultaneously proximinal in L-1 (mu, X) in the sense of Fathi et al. [Best simultaneous approximation in L-p(I, E), J. Approx. Theory 116 (2002), 369-379]. Some examples and remarks related with N-simultaneous proximinality are also given.
