A new combinatorial characterization of the minimal cardinality of a subset of R which is not of first category

Apoloniusz Tyszka

Published 1999-11-17, updated 1999-11-30Version 2

Let M denote the ideal of first category subsets of R. We prove that min{card X: X \subseteq R, X \not\in M} is the smallest cardinality of a family S \subseteq {0,1}^\omega with the property that for each f: \omega -> \bigcup_{n \in \omega}{0,1}^n there exists a sequence {a_n}_{n \in \omega} belonging to S such that for infinitely many i \in \omega the infinite sequence {a_{i+n}}_{n \in \omega} extends the finite sequence f(i). We inform that S \subseteq {0,1}^\omega is not of first category if and only if for each f: \omega -> \bigcup_{n \in \omega}{0,1}^n there exists a sequence {a_n}_{n \in \omega} belonging to S such that for infinitely many i \in \omega the infinite sequence {a_{i+n}}_{n \in \omega} extends the finite sequence f(i).