English

Connected and/or topological group pd-examples

General Topology 2017-05-09 v1

Abstract

The pinning down number pd(X)pd(X) of a topological space XX is the smallest cardinal κ\kappa such that for every neighborhood assignment U\mathcal{U} on XX there is a set of size κ\kappa that meets every member of U\mathcal{U}. Clearly, pd(X)d(X)pd(X) \le d(X) and we call XX a pd-example if pd(X)<d(X)pd(X) < d(X). We denote by S\mathbf{S} the class of all singular cardinals that are not strong limit. It was proved in a paper of Juh\'asz,Soukup and Szentmikl\'ossy (arXiv:1506.00206}) that TFAE: (1) S\mathbf{S} \ne \emptyset; (2) there is a 0-dimensional T2T_2 pd-example; (3) there is a T2T_2 pd-example. The aim of this paper is to produce pd-examples with further interesting topological properties like connectivity or being a topological group by presenting several constructions that transform given pd-examples into ones with these additional properties. We show that S\mathbf{S} \ne \emptyset is also equivalent to the existence of a connected and locally connected T3T_3 pd-example, as well as to the existence of an abelian T2T_2 topological group pd-example. However, S\mathbf{S} \ne \emptyset in itself is not sufficient to imply the existence of a connected T3.5T_{3.5} pd-example. But if there is μS\mu \in \mathbf{S} with μc\mu \ge \mathfrak{c} then there is an abelian T2T_2 topological group (hence T3.5T_{3.5}) pd-example which is also arcwise connected and locally arcwise connected. Finally, the same assumption Sc\,\mathbf{S} \setminus \mathfrak{c} \ne \emptyset\, even implies that there is a locally convex topological vector space pd-example.

Keywords

Cite

@article{arxiv.1705.02622,
  title  = {Connected and/or topological group pd-examples},
  author = {Istvan Juhász and Jan van Mill and Lajos Soukup and Zoltán Szentmiklóssy},
  journal= {arXiv preprint arXiv:1705.02622},
  year   = {2017}
}

Comments

14 pages

R2 v1 2026-06-22T19:39:31.660Z