A metric interpretation of reflexivity for Banach spaces

TL;DR

This paper introduces metrics on Schreier families to characterize Banach space reflexivity and relates Szlenk index bounds to bi-Lipschitz embeddings of these metric spaces.

Contribution

It provides a metric-based characterization of reflexivity and links Szlenk index bounds to bi-Lipschitz embeddability of Schreier family metric spaces.

Findings

Reflexivity characterized by absence of certain bi-Lipschitz embeddings.

Metrics $d_{1, extalpha}$ and $d_{ extalpha}$ on Schreier families are key tools.

Szlenk index bounds correspond to non-embeddability of Schreier spaces.

Abstract

We define two metrics $d_{1,\alpha}$ and $d_{\infty,\alpha}$ on each Schreier family $\mathcal{S}_\alpha$, $\alpha<\omega_1$, with which we prove the following metric characterization of reflexivity of a Banach space $X$: $X$ is reflexive if and only if there is an $\alpha<\omega_1$, so that there is no mapping $\Phi:\mathcal{S}_\alpha\to X$ for which $$ cd_{\infty,\alpha}(A,B)\le \|\Phi(A)-\Phi(B)\|\le C d_{1,\alpha}(A,B) \text{ for all $A,B\in\mathcal{S}_\alpha$.}$$ Secondly, we prove for separable and reflexive Banach spaces $X$, and certain countable ordinals $\alpha$ that $\max(\text{ Sz}(X),\text{ Sz}(X^*))\le \alpha$ if and only if $({\mathcal S}_\alpha, d_{1,\alpha})$ does not bi-Lipschitzly embed into $X$. Here $\text{Sz}(Y)$ denotes the Szlenk index of a Banach space $Y$.