Tameness and Rosenthal type locally convex spaces

Abstract

Motivated by Rosenthal's famous $l^1$-dichotomy in Banach spaces, Haydon's theorem, and additionally by recent works on tame dynamical systems, we introduce the class of tame locally convex spaces. This is a natural locally convex analogue of Rosenthal Banach spaces (for which any bounded sequence contains a weak Cauchy subsequence). Our approach is based on a bornology of tame subsets which in turn is closely related to eventual fragmentability. This leads, among others, to the following results: $\bullet$ extending Haydon's characterization of Rosenthal Banach spaces, by showing that a lcs $E$ is tame iff every weak-star compact, equicontinuous convex subset of $E^{*}$ is the strong closed convex hull of its extreme points iff $\overline{\rm{co\,}}^{w^{*}}(K) = \overline{\rm{co\,}}(K)$ for every weak-star compact equicontinuous subset $K$ of $E^{*}$; $\bullet$ $E$ is tame iff there is no bounded sequence equivalent to the generalized $l^{1}$-sequence; $\bullet$ strengthening some results of W.M. Ruess about Rosenthal's dichotomy; $\bullet$ applying the Davis-Figiel-Johnson-Pelczy\'nski (DFJP) technique one may show that every tame operator $T \colon E \to F$ between a lcs $E$ and a Banach space $F$ can be factored through a tame (i.e., Rosenthal) Banach space.