On the cardinality of Hausdorff spaces and H-closed spaces

TL;DR

This paper introduces a new cardinal invariant for Hausdorff spaces, providing improved bounds on their cardinality and exploring generalized closedness properties related to ultrafilters and filters.

Contribution

It defines the invariant $aL^\'(X)$, establishes new bounds on space cardinalities, and introduces generalized closedness notions like $ abla$wH-closed spaces.

Findings

$|X| extless= 2^{aL^\'(X)\chi(X)}$ for Hausdorff spaces

$aL^\'(X)$ equals $ ext{aleph}_0$ for H-closed spaces

Generalizations of H-closed spaces with cardinality bounds

Abstract

We introduce the cardinal invariant $aL^\prime(X)$ and show that $|X|\leq 2^{aL^\prime(X)\chi(X)}$ for any Hausdorff space $X$ (a corollary of Theorem 4.4. This invariant has the properties a) $aL^\prime(X)=\aleph_0$ if $X$ is H-closed, and b) $aL(X)\leq aL^\prime(X)\leq aL_c(X)$. Theorem 4.4 then gives a new improvement of the well-known Hausdorff bound $2^{L(X)\chi(X)}$ from which it follows that $|X|\leq 2^{\psi_c(X)}$ if $X$ is H-closed (Dow/Porter [5]). The invariant $aL^\prime(X)$ is constructed using convergent open ultrafilters and an operator $c:\scr{P}(X)\to\scr{P}(X)$ with the property $clA\subseteq c(A)\subseteq cl_\theta(A)$ for all $A\subseteq X$. As a comparison with this open ultrafilter approach, in $\S 3$ we additionally give a $\kappa$-filter variation of Hodel's proof [10] of the Dow-Porter result. Finally, for an infinite cardinal $\kappa$, in $\S 5$ we introduce $\kappa$wH-closed spaces, $\kappa H^\prime$-closed spaces, and $\kappa H^{\prime\prime}$-closed spaces. The first two notions generalize the H-closed property. Key results in this connection are that a) if $\kappa$ is an infinite cardinal and $X$ a $\kappa$wH-closed space with a dense set of isolated points such that $\chi(X)\leq\kappa$, then $|X|\leq 2^{\kappa}$, and b) if $X$ is $\kappa H^\prime$-closed or $\kappa H^{\prime\prime}$-closed then $aL^\prime(X)\leq\kappa$. This latter result relates these notions to the invariant $aL^\prime(X)$ and the operator $c$.