Large strongly anti-Urysohn spaces exist

TL;DR

This paper constructs large strongly anti-Urysohn (SAU) spaces within ZFC, answering key questions about their existence and size, and explores consistency results for crowded SAU spaces under various set-theoretic assumptions.

Contribution

It provides the first ZFC construction of a large locally countable SAU space of size 2^c and investigates the existence of crowded SAU spaces through consistency results.

Findings

Constructed a locally countable SAU space of size 2^c in ZFC.

Showed the existence of crowded SAU spaces of size c^+ and 2^c under certain set-theoretic assumptions.

Established equivalences involving the existence of crowded SAU spaces and cofinality conditions.

Abstract

As defined in [1], a Hausdorff space is strongly anti-Urysohn (in short: SAU) if it has at least two non-isolated points and any two infinite} closed subsets of it intersect. Our main result answers the two main questions of [1] by providing a ZFC construction of a locally countable SAU space of cardinality $2^{\mathfrak{c}}$. The construction hinges on the existence of $2^{\mathfrak{c}}$ weak P-points in $\omega^*$, a very deep result of Ken Kunen. It remains open if SAU spaces of cardinality $> 2^{\mathfrak{c}}$ could exist, while it was shown in [1] that $2^{2^{\mathfrak{c}}}$ is an upper bound. Also, we do not know if crowded SAU spaces, i.e. ones without any isolated points, exist in ZFC but we obtained the following consistency results concerning such spaces. (1) It is consistent that $\mathfrak{c}$ is as large as you wish and there is a locally countable and crowded SAU space of cardinality $\mathfrak{c}^+$. (2) It is consistent that both $\mathfrak{c}$ and $2^\mathfrak{c}$ are as large as you wish and there is a crowded SAU space of cardinality $2^\mathfrak{c}$. (3) For any uncountable cardinal ${\kappa}$ the following statements are equivalent: (i) ${\kappa}=cof({[{\kappa}]}^{{\omega}},\subseteq)$. (ii) There is a locally countable and crowded SAU space of size ${\kappa}$ in the generic extension obtained by adding $\kappa$ Cohen reals. (iii) There is a locally countable and countably compact $T_1$-space of size ${\kappa}$ in some CCC generic extension. [1] I. Juhasz, L. Soukup, and Z. Szentmiklossy, Anti-Urysohn spaces, Top. Appl., 213 (2016), pp. 8--23.