Transitivity of the norm on Banach spaces¶ having a Jordan structure

Abstract

We study transitivity conditions on the norm of JB -triples, C-algebras, JB - algebras, and their preduals. We show that, for the predual X of a JBW -triple, each one of the following conditions i) and ii) implies that X is a Hilbert space. i) The closed unit ball of X has some extreme point and the norm of X is convex transitive. ii) The set of all extreme points of the closed unit ball of X is non rare in the unit sphere of X. These results are applied to obtain partial affirmative answers to the open problem whether every JB -triple with transitive norm is a Hilbert space. We extend to arbitrary C-algebras previously known characterizations of transitivity (20) and convex transitivity (36) of the norm on commutative C-algebras. Moreover, we prove that the Calkin algebra has convex transitive norm. We also prove that, if X is a JB -algebra, and if either the norm of X is convex transitive or X has a predual with convex transitive norm, then X is associative. As a consequence, a JB -algebra with almost transitive norm is isomorphic to the field of real numbers.