English

Between countably compact and $\omega$-bounded

General Topology 2014-07-01 v1

Abstract

Given a property PP of subspaces of a T1T_1 space XX, we say that XX is {\em PP-bounded} iff every subspace of XX with property PP has compact closure in XX. Here we study PP-bounded spaces for the properties P{ωD,ωN,C2}P \in \{\omega D, \omega N, C_2 \} where ωD\omega D \, \equiv "countable discrete", ωN\omega N \, \equiv "countable nowhere dense", and C2C_2 \,\equiv "second countable". Clearly, for each of these PP-bounded is between countably compact and ω\omega-bounded. We give examples in ZFC that separate all these boundedness properties and their appropriate combinations. Consistent separating examples with better properties (such as: smaller cardinality or weight, local compactness, first countability) are also produced. We have interesting results concerning ωD\omega D-bounded spaces which show that ωD\omega D-boundedness is much stronger than countable compactness: \bullet Regular ωD\omega D-bounded spaces of Lindel\"of degree <cov(M)< cov(\mathcal{M}) are ω\omega-bounded. \bullet Regular ωD\omega D-bounded spaces of countable tightness are ωN\omega N-bounded, and if b>ω1\mathfrak{b} > \omega_1 then even ω\omega-bounded. \bullet If a product of Hausdorff space is ωD\omega D-bounded then all but one of its factors must be ω\omega-bounded. \bullet Any product of at most t\mathfrak{t} many Hausdorff ωD\omega D-bounded spaces is countably compact. As a byproduct we obtain that regular, countably tight, and countably compact spaces are discretely generated.

Keywords

Cite

@article{arxiv.1406.7805,
  title  = {Between countably compact and $\omega$-bounded},
  author = {István Juhász and Lajos Soukup and Zoltán Szentmiklóssy},
  journal= {arXiv preprint arXiv:1406.7805},
  year   = {2014}
}

Comments

14 pages

R2 v1 2026-06-22T04:51:32.981Z