The minimax principle and related topics in the Jordan setting

Abstract

We prove a minimax principle for weakly compact JB${}^{\ast }$-triples characterizing geometrically the singular values of an element. Among the consequences of this principle we present a Weyl inequality on the perturbation of the singular values and a Cauchy-Poincaré (interlacing) theorem. We also obtain a version of the Ky Fan maximum principle in the setting of weakly compact JB${}^{\ast }$-triples. We study perturbations of the spectral resolutions showing that small perturbations of an element produces small perturbations of the corresponding spectral resolutions. As a consequence we obtain that weakly compact JB${}^{\ast }$-triples satisfy the property that perturbations of a convex combination of elements in the closed unit ball coincide with a convex combination of perturbations of the elements also in the closed unit ball. All these results hold true when particularized to weakly compact JB${}^{\ast }$-algebras.