English

Weak Baumgartner axioms and universal spaces

Logic 2025-03-11 v2 General Topology

Abstract

If XX is a topological space and κ\kappa is a cardinal then BAκ(X)\mathsf{BA}_\kappa (X) is the statement that for each pair A,BXA, B \subseteq X of κ\kappa-dense subsets there is an autohomeomorphism h:XXh:X \to X mapping AA to BB. In particular BA1(R)\mathsf{BA}_{\aleph_1} (\mathbb R) is equivalent the celebrated Baumgartner axiom on isomorphism types of 1\aleph_1-dense linear orders. In this paper we consider two natural weakenings of BAκ(X)\mathsf{BA}_\kappa (X) which we call BAκ(X)\mathsf{BA}^-_\kappa (X) and Uκ(X)\mathsf{U}_\kappa (X) for arbitrary perfect Polish spaces XX. We show that the first of these, though properly weaker, entails many of the more striking consequences of BAκ(X)\mathsf{BA}_\kappa (X) while the second does not. Nevertheless the second is still independent of ZFC\mathsf{ZFC} and we show in particular that it fails in the Cohen and random models. This motivates several new classes of pairs of spaces which are ``very far from being homeomorphic" which we call ``avoiding", ``strongly avoiding", and ``totally avoiding". The paper concludes by studying these classes, particularly in the context of forcing theory, in an attempt to gauge how different weak Baumgartner axioms may be separated.

Keywords

Cite

@article{arxiv.2502.10029,
  title  = {Weak Baumgartner axioms and universal spaces},
  author = {Corey Bacal Switzer},
  journal= {arXiv preprint arXiv:2502.10029},
  year   = {2025}
}

Comments

20 pages, submitted. Second version incorporates several recommendations from anonymous referees. In particular the terminology of "hating" etc has been changed

R2 v1 2026-06-28T21:44:14.072Z