A new order for ideal sequential compactness

Abstract

Let $\mathcal{I}$ be an ideal on $\ddot{I}‰$ and $X$ be a topological space. A sequence $(x_n{)}_{n\in \ddot{I}‰}$ in $X$ is $\mathcal{I}$-convergent if there is $x\in X$ such that $\{n\in \ddot{I}‰:x_n\notin U\}\in \mathcal{I}$ for every open neighborhood $U$ of $x$. We examine the following variant of sequential compactness associated with $\I$: $X$ is $\mathrm{BW}(\mathcal{I})$ if for every sequence $(x_n{)}_{n\in \ddot{I}‰}$ in $X$ there is $A\notin \mathcal{I}$ such that $(x_n{)}_{n\in A}$ is $\mathcal{I}$-convergent. We introduce a new preorder on ideals, denoted ${\le }_{BW}$, such that $\mathcal{I}{\le }_{BW}\mathcal{J}$ implies that every $\mathrm{BW}(\mathcal{J})$ space is $\mathrm{BW}(\mathcal{I})$. Our main result states that under CH the above implication can be reversed in the case of ${\mathbf{F}}_{\ddot{\mathbf{I}}}ƒ$ ideals $\I$ and $\J$. We compare ${\le }_{BW}$ with the Katětov order and study the relation ${\le }_{BW}$ among some well-known ideals (e.g. the van der Waerden ideal $\mathcal{W}$ consisting of all subsets of $\ddot{I}‰$ that do not contain arbitrary long finite arithmetic progressions). As a consequence, we answer two open questions posed by Filipów and Tryba in [Top. App. {\textbf{178}} (2014), 438--452] concerning comparison of $\mathrm{BW}(\mathcal{W})$ with the class of sequentially compact spaces.