The set of bounded continuous nowhere locally uniformly continuous functions is not Borel

TL;DR

This paper investigates the descriptive set-theoretic complexity of bounded continuous nowhere locally uniformly continuous functions on various metric spaces, revealing that nonseparable spaces lead to non-Borel sets.

Contribution

It demonstrates that for nonseparable metric spaces, the set of such functions is not Borel, contrasting with the separable case where it is a $G_\delta$ set.

Findings

In separable spaces, the set is a $G_\delta$ set.

In nonseparable spaces, the set is not Borel.

The set's complexity depends on the separability of the space.

Abstract

It is known that for $X$ a nowhere locally compact metric space, the set of bounded continuous, nowhere locally uniformly continuous real-valued functions on $X$ contains a dense $G_\delta$ set in the space $C_b(X)$ of all bounded continuous real-valued functions on $X$ in the supremum norm. Furthermore, when $X$ is separable, the set of bounded continuous, nowhere locally uniformly continuous real-valued functions on $X$ is itself a $G_\delta$ set. We show that in contrast, when $X$ is nonseparable, this set of functions is not even a Borel set.