On a combinatorial property of families of sequences converging to +infinity

Abstract

We consider families F of sequences converging to +infinity that F satisfies the following condition (C): (C): if an open set U in the real line is unbounded above then there exists a sequence belonging to F, which has an infinite number of terms belonging to U. For the functions f,g from {0,1,2,...} to {0,1,2,...} we define: f =< g if and only if {i: f(i) > g(i)} is finite. Let b denote the smallest cardinality of a unbounded (in the sense of =<) family of functions from {0,1,2,...} to {0,1,2,...}, see [1]. Theorem 1. If F is a family of sequences converging to +infinity and card F < b, then F does not satisfy condition (C). Corollary. Every family of sequences converging to +infinity which satisfies the condition (C) is uncountable, Martin&#39;s axiom implies that each such family has cardinality continuum (because Martin&#39;s axiom implies that b=continuum, see [1], [2]). Theorem 2. There exists a family of sequences converging to +infinity which satisfies condition (C) and has cardinality b. References [ 1 ] R. Frankiewicz, P. Zbierski, Hausdorff gaps and limits, Amsterdam: North-Holland, 1994. [ 2 ] T. Jech, Set theory, New York, Academic Press, 1978.