English

Dominating and pinning down pairs for topological spaces

General Topology 2020-11-23 v1 Logic

Abstract

We call a pair of infinite cardinals (κ,λ)(\kappa,\lambda) with κ>λ\kappa > \lambda a dominating (resp. pinning down) pair for a topological space XX if for every subset AA of XX (resp. family U\mathcal{U} of non-empty open sets in XX) of cardinality κ\le \kappa there is BXB \subset X of cardinality λ\le \lambda such that ABA \subset \overline{B} (resp. BUB \cap U \ne \emptyset for each UUU \in \mathcal{U}). Clearly, a dominating pair is also a pinning down pair for XX. Our definitions generalize the concepts introduced in [GTW] resp. [BT] which focused on pairs of the form (2λ,λ)(2^\lambda,\lambda). The main aim of this paper is to answer a large number of the numerous problems from [GTW] and [BT] that asked if certain conditions on a space XX together with the assumption that (2λ,λ)(2^\lambda,\lambda) or ((2λ)+,λ)((2^\lambda)^+,\lambda) is a pinning down pair or \dominating pair for XX would imply d(X)λd(X) \le \lambda. [BT] A. Bella, V.V. Tkachuk, Exponential density vs exponential domination, preprint [GTW] G. Gruenhage, V.V. Tkachuk, R.G. Wilson, Domination by small sets versus density, Topology and its Applications 282 (2020)

Keywords

Cite

@article{arxiv.2011.10261,
  title  = {Dominating and pinning down pairs for topological spaces},
  author = {István Juhász and Lajos Soukup and Zoltán Szentmiklóssy},
  journal= {arXiv preprint arXiv:2011.10261},
  year   = {2020}
}

Comments

16 pages

R2 v1 2026-06-23T20:23:24.012Z