Uncountable equilateral sets in Banach spaces of the form $C(K)$

TL;DR

Under certain set-theoretic assumptions, every nonseparable Banach space of the form C(K) contains an uncountable equilateral set, but this is not provable without additional assumptions, and the existence of such sets varies with renormings.

Contribution

The paper demonstrates that set-theoretic axioms influence the existence of uncountable equilateral sets in C(K) spaces and constructs examples showing the independence of this property.

Findings

Martin's axiom implies uncountable equilateral sets in C(K) spaces.

Counterexamples show the necessity of additional set-theoretic assumptions.

Existence of uncountable equilateral sets can depend on renormings of the space.

Abstract

The paper is concerned with the problem whether a nonseparable Banach space must contain an uncountable set of vectors such that the distances between every two distinct vectors of the set are the same. Such sets are called equilateral. We show that Martin's axiom and the negation of the continuum hypothesis imply that every nonseparable Banach space of the form $C(K)$ has an uncountable equilateral set. We also show that one cannot obtain such a result without an additional set-theoretic assumption since we construct an example of nonseparable Banach space of the form $C(K)$ which has no uncountable equilateral set (or equivalently no uncountable $(1+\varepsilon)$-separated set in the unit sphere for any $\varepsilon>0$) making another consistent combinatorial assumption. The compact $K$ is a version of the split interval obtained from a sequence of functions which behave in an anti-Ramsey manner. It remains open if there is an absolute example of a nonseparable Banach space of the form different than $C(K)$ which has no uncountable equilateral set. It follows from the results of S. Mercourakis, G. Vassiliadis that our example has an equivalent renorming in which it has an uncountable equilateral set. It remains open if there are consistent examples which have no uncountable equilateral sets in any equivalent renorming but it follows from the results of S. Todorcevic that it is consistent that every nonseparable Banach space has an equivalent renorming in which it has an uncountable equilateral set.