Spaces of countable free set number and PFA

TL;DR

This paper investigates the size and weight of regular spaces with countable free set number under PFA, establishing bounds and providing counterexamples in ZFC, including a strongly anti-Urysohn space with no isolated points.

Contribution

It proves under PFA that regular spaces with countable free set number have size bounds related to their weight, and constructs a ZFC example of a Hausdorff SAU space with no isolated points.

Findings

Under PFA, regular spaces with F(X)=ω have size ≤ w(X)^ω.

Constructs a ZFC Hausdorff space with F(X)=ω, w(X)=c, and size 2^c.

Provides a ZFC example of a SAU space with no isolated points.

Abstract

The main result of this paper is that, under PFA, for every {\em regular} space $X$ with $F(X) = \omega$ we have $|X| \le w(X)^\omega$; in particular, $w(X) \le \mathfrak{c}$ implies $|X| \le \mathfrak{c}$. This complements numerous prior results that yield consistent examples of even compact Hausdorff spaces $X$ with $F(X) = \omega$ such that $w(X) = \mathfrak{c}$ and $|X| = 2^\mathfrak{c}$. We also show that regularity cannot be weakened to Hausdorff in this result because we can find in ZFC a Hausdorff space $X$ with $F(X) = \omega$ such that $w(X) = \mathfrak{c}$ and $|X| = 2^\mathfrak{c}$. In fact, this space $X$ has the {\em strongly anti-Urysohn} (SAU) property that any two infinite closed sets in $X$ intersect, which is much stronger than $F(X) = \omega$. Moreover, any non-empty open set in $X$ also has size $2^\mathfrak{c}$, and thus answers one of the main problems of \cite{JShSSz} by providing in ZFC a SAU space with no isolated points.