Countable compactness and $p$-limits

García-Ferreira, S. ; Tomita, A. H.

Commentationes Mathematicae Universitatis Carolinae, Tome 42 (2001) no. 3, pp. 521-527

Résumé

For $\emptyset \neq M \subseteq \omega^*$, we say that $X$ is quasi $M$-compact, if for every $f: \omega \rightarrow X$ there is $p \in M$ such that $\overline{f}(p) \in X$, where $\overline{f}$ is the Stone-Čech extension of $f$. In this context, a space $X$ is countably compact iff $X$ is quasi $\omega^*$-compact. If $X$ is quasi $M$-compact and $M$ is either finite or countable discrete in $\omega^*$, then all powers of $X$ are countably compact. Assuming $CH$, we give an example of a countable subset $M \subseteq \omega^*$ and a quasi $M$-compact space $X$ whose square is not countably compact, and show that in a model of A. Blass and S. Shelah every quasi $M$-compact space is $p$-compact (= quasi $\{p\}$-compact) for some $p \in \omega^*$, whenever $M \in [\omega^*]^{ {\frak c}}$. We prove that if $\emptyset \notin \{ T_\xi :\, \xi 2^{{\frak c}} \} \subseteq [\omega^*]^{ 2^{{\frak c}}}$, then there is a countably compact space $X$ that is not quasi $T_\xi$-compact for every $\xi 2^{{\frak c}}$; hence, if $2^{{\frak c}}$ is regular, then there is a countably compact space $X$ such that $X$ is not quasi $M$-compact for any $M \in [\omega^*]^{ 2^{{\frak c}}}$. We list some open problems.

Détail

Classification : 54A20, 54A35, 54B99, 54D20, 54D30
Keywords: $p$-limit; $p$-compact; almost $p$-compact; quasi $M$-compact; countably compact