Separated sets and Auerbach systems in Banach spaces

TL;DR

This paper explores the relationship between the density of Banach spaces and the size of well-separated subsets of their unit spheres, introducing new results on Auerbach systems and their existence in various classes of Banach spaces.

Contribution

It establishes new sharp results on the existence and size of Auerbach systems and separated subsets in Banach spaces, including the first example of a non-separable WLD space without uncountable Auerbach systems.

Findings

Uncountable separated subsets exist in large Banach spaces.

Constructed a non-separable WLD space with no uncountable Auerbach system.

Proved optimal separated subset sizes in reflexive and super-reflexive spaces.

Abstract

The paper elucidates the relationship between the density of a Banach space and possible sizes of well-separated subsets of its unit sphere. For example, it is proved that for a large enough space $X$, the unit sphere $S_X$ always contains an uncountable $(1+)$-separated subset. In order to achieve this, new results concerning the existence of large Auerbach systems are established that happen to be sharp for the class of WLD spaces. In fact, we offer the first consistent example of a non-separable WLD Banach space that contains no uncountable Auerbach system, as witnessed by a renorming of $c_0(\omega_1)$. Moreover, the following optimal results for the classes of, respectively, reflexive and super-reflexive spaces are established: the unit sphere of an infinite-dimensional reflexive space contains a symmetrically $(1+\varepsilon)$-separated subset of any regular cardinality not exceeding the density of $X$; should the space $X$ be super-reflexive, the unit sphere of $X$ contains such a subset of cardinality equal to the density of $X$. The said problem is studied for other classes of spaces too, including the RNP spaces or strictly convex ones.