English

Anti-Urysohn spaces

General Topology 2015-09-07 v1 Logic

Abstract

All spaces are assumed to be infinite Hausdorff spaces. We call a space "anti-Urysohn" ((AU in short)) iff any two non-emty regular closed sets in it intersect. We prove that \bullet for every infinite cardinal κ{\kappa} there is a space of size κ{\kappa} in which fewer than cf(κ)cf({\kappa}) many non-empty regular closed sets always intersect; \bullet there is a locally countable AU space of size κ\kappa iff ωκ2c\omega \le \kappa \le 2^{\mathfrak c}. A space with at least two non-isolated points is called "strongly anti-Urysohn" ((SAU in short)) iff any two infinite closed sets in it intersect. We prove that \bullet if XX is any SAU space then sX22c \mathfrak s\le |X|\le 2^{2^{\mathfrak c}}; \bullet if r=c\mathfrak r=\mathfrak c then there is a separable, crowded, locally countable, SAU space of cardinality c\mathfrak c; \item if λ>ω\lambda > \omega Cohen reals are added to any ground model then in the extension there are SAU spaces of size κ\kappa for all κ[ω1,λ]\kappa \in [\omega_1,\lambda]; \bullet if GCH holds and κλ\kappa \le\lambda are uncountable regular cardinals then in some CCC generic extension we have s=κ\mathfrak s={\kappa}, c=λ\,\mathfrak c={\lambda}, and for every cardinal μ[s,c]{\mu}\in [\mathfrak s, \mathfrak c] there is an SAU space of cardinality μ{\mu}. The questions if SAU spaces exist in ZFC or if SAU spaces of cardinality >c> \mathfrak c can exist remain open.

Keywords

Cite

@article{arxiv.1509.01420,
  title  = {Anti-Urysohn spaces},
  author = {István Juhász and Lajos Soukup and Zoltán Szentmiklóssy},
  journal= {arXiv preprint arXiv:1509.01420},
  year   = {2015}
}
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