English

A new order for ideal sequential compactness

General Topology 2026-03-03 v1

Abstract

Let I\mathcal{I} be an ideal on ω\omega and XX be a topological space. A sequence (xn)nω(x_n)_{n\in \omega} in XX is I\mathcal{I}-convergent if there is xXx\in X such that {nω:xnU}I\{n\in \omega:x_n\notin U\}\in\mathcal{I} for every open neighborhood UU of xx. We examine the following variant of sequential compactness associated with \I\I: XX is BW(I)\mathrm{BW}(\mathcal{I}) if for every sequence (xn)nω(x_n)_{n\in \omega} in XX there is AIA\notin\mathcal{I} such that (xn)nA(x_n)_{n\in A} is I\mathcal{I}-convergent. We introduce a new preorder on ideals, denoted BW\leq_{BW}, such that IBWJ\mathcal{I}\leq_{BW}\mathcal{J} implies that every BW(J)\mathrm{BW}(\mathcal{J}) space is BW(I)\mathrm{BW}(\mathcal{I}). Our main result states that under CH the above implication can be reversed in the case of Fσ\mathbf{F_\sigma} ideals \I\I and \J\J. We compare BW\leq_{BW} with the Kat\v{e}tov order and study the relation BW\leq_{BW} among some well-known ideals (e.g. the van der Waerden ideal W\mathcal{W} consisting of all subsets of ω\omega that do not contain arbitrary long finite arithmetic progressions). As a consequence, we answer two open questions posed by Filip\'{o}w and Tryba in [Top. App. {\textbf{178}} (2014), 438--452] concerning comparison of BW(W)\mathrm{BW}(\mathcal{W}) with the class of sequentially compact spaces.

Keywords

Cite

@article{arxiv.2603.01114,
  title  = {A new order for ideal sequential compactness},
  author = {Adam Kwela and Dorota Lesner},
  journal= {arXiv preprint arXiv:2603.01114},
  year   = {2026}
}
R2 v1 2026-07-01T10:57:59.511Z