English

Variations on known and recent cardinality bounds

General Topology 2017-10-02 v1

Abstract

Sapirovskii [18] proved that Xπχ(X)c(X)ψ(X)|X|\leq\pi\chi(X)^{c(X)\psi(X)}, for a regular space XX. We introduce the θ\theta-pseudocharacter of a Urysohn space XX, denoted by ψθ(X)\psi_\theta (X), and prove that the previous inequality holds for Urysohn spaces replacing the bounds on celluarity c(X)κc(X)\leq\kappa and on pseudocharacter ψ(X)κ\psi(X)\leq\kappa with a bound on Urysohn cellularity Uc(X)κUc(X)\leq\kappa (which is a weaker conditon because Uc(X)c(X)Uc(X)\leq c(X)) and on θ\theta-pseudocharacter ψθ(X)κ\psi_\theta (X)\leq\kappa respectivly (note that in general ψ()ψθ()\psi(\cdot)\leq\psi_\theta (\cdot) and in the class of regular spaces ψ()=ψθ()\psi(\cdot)=\psi_\theta(\cdot)). Further, in [6] the authors generalized the Dissanayake and Willard's inequality: X2aLc(X)χ(X)|X|\leq 2^{aL_{c}(X)\chi(X)}, for Hausdorff spaces XX [25], in the class of nn-Hausdorff spaces and de Groot's result: X2hL(X)|X|\leq 2^{hL(X)}, for Hausdorff spaces [11], in the class of T1T_1 spaces (see Theorems 2.22 and 2.23 in [6]). In this paper we restate Theorem 2.22 in [6] in the class of nn-Urysohn spaces and give a variation of Theorem 2.23 in [6] using new cardinal functions, denoted by UW(X)UW(X), ψwθ(X)\psi w_\theta(X), θ-aL(X)\theta\hbox{-}aL(X), hθ-aL(X)h\theta\hbox{-}aL(X), θ-aLc(X)\theta\hbox{-}aL_c(X) and θ-aLθ(X)\theta\hbox{-}aL_{\theta}(X). In [5] the authors introduced the Hausdorff point separating weight of a space XX denoted by Hpsw(X)Hpsw(X) and proved a Hausdorff version of Charlesworth's inequality Xpsw(X)L(X)ψ(X)|X|\leq psw(X)^{L(X)\psi(X)} [7]. In this paper, we introduce the Urysohn point separating weight of a space XX, denoted by Upsw(X)Upsw(X), and prove that XUpsw(X)θ-aLc(X)ψ(X)|X|\leq Upsw(X)^{\theta\hbox{-}aL_{c}(X)\psi(X)}, for a Urysohn space XX.

Cite

@article{arxiv.1709.10497,
  title  = {Variations on known and recent cardinality bounds},
  author = {Fortunata Aurora Basile and Maddalena Bonanzinga and Nathan Carlson},
  journal= {arXiv preprint arXiv:1709.10497},
  year   = {2017}
}

Comments

14 pages

R2 v1 2026-06-22T21:59:10.889Z