Characterizations of $\mathbb{N}$-compactness and realcompactness via ultrafilters in the absence of the axiom of choice

Abstract: This article concerns the Herrlich-Chew theorem stating that a Hausdorff zero-dimensional space is $\mathbb{N}$-compact if and only if every clopen ultrafilter with the countable intersection property in this space is fixed. It also concerns Hewitt's theorem stating that a Tychonoff space is realcompact if and only if every $z$-ultrafilter with the countable intersection property in this space is fixed. The axiom of choice was involved in the original proofs of these theorems. The aim of this article is to show that the Herrlich-Chew theorem is valid in $\mathbf{ZF}$, but it is an open problem if Hewitt's theorem can be false in a model of $\mathbf{ZF}$. It is proved that Hewitt's theorem is true in every model of $\mathbf{ZF}$ in which the countable axiom of multiple choice is satisfied. A modification of Hewitt's theorem is given and proved true in $\mathbf{ZF}$. Several applications of the results obtained are shown.