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Variations on known and recent cardinality bounds

Research paper by Fortunata Aurora Basile, Maddalena Bonanzinga, Nathan Carlson

Indexed on: 29 Sep '17Published on: 29 Sep '17Published in: arXiv - Mathematics - General Topology



Abstract

Sapirovskii [18] proved that $|X|\leq\pi\chi(X)^{c(X)\psi(X)}$, for a regular space $X$. We introduce the $\theta$-pseudocharacter of a Urysohn space $X$, denoted by $\psi_\theta (X)$, and prove that the previous inequality holds for Urysohn spaces replacing the bounds on celluarity $c(X)\leq\kappa$ and on pseudocharacter $\psi(X)\leq\kappa$ with a bound on Urysohn cellularity $Uc(X)\leq\kappa$ (which is a weaker conditon because $Uc(X)\leq c(X)$) and on $\theta$-pseudocharacter $\psi_\theta (X)\leq\kappa$ respectivly (note that in general $\psi(\cdot)\leq\psi_\theta (\cdot)$ and in the class of regular spaces $\psi(\cdot)=\psi_\theta(\cdot)$). Further, in [6] the authors generalized the Dissanayake and Willard's inequality: $|X|\leq 2^{aL_{c}(X)\chi(X)}$, for Hausdorff spaces $X$ [25], in the class of $n$-Hausdorff spaces and de Groot's result: $|X|\leq 2^{hL(X)}$, for Hausdorff spaces [11], in the class of $T_1$ spaces (see Theorems 2.22 and 2.23 in [6]). In this paper we restate Theorem 2.22 in [6] in the class of $n$-Urysohn spaces and give a variation of Theorem 2.23 in [6] using new cardinal functions, denoted by $UW(X)$, $\psi w_\theta(X)$, $\theta\hbox{-}aL(X)$, $h\theta\hbox{-}aL(X)$, $\theta\hbox{-}aL_c(X)$ and $\theta\hbox{-}aL_{\theta}(X)$. In [5] the authors introduced the Hausdorff point separating weight of a space $X$ denoted by $Hpsw(X)$ and proved a Hausdorff version of Charlesworth's inequality $|X|\leq psw(X)^{L(X)\psi(X)}$ [7]. In this paper, we introduce the Urysohn point separating weight of a space $X$, denoted by $Upsw(X)$, and prove that $|X|\leq Upsw(X)^{\theta\hbox{-}aL_{c}(X)\psi(X)}$, for a Urysohn space $X$.