The non-Urysohn number of a topological space

Abstract

We call a nonempty subset $A$ of a topological space $X$ finitely non-Urysohn if for every nonempty finite subset $F$ of $A$ and every family $\{U_x:x\in F\}$ of open neighborhoods $U_x$ of $x\in F$, $\cap\{\mathrm{cl}(U_x):x\in F\}\ne\emptyset$ and we define the non-Urysohn number of $X$ as follows: $nu(X):=1+\sup\{|A|:A$ is a finitely non-Urysohn subset of $X\}$. Then for any topological space $X$ and any subset $A$ of $X$ we prove the following inequalities: (1) $|\mathrm{cl}_θ(A)|\le |A|^{κ(X)}\cdot nu(X)$, (2) $|[A]_θ|\le (|A|\cdot nu(X))^{κ(X)}$, (3) $|X|\le nu(X)^{κ(X)sL_θ(X)}$, and (4) $|X|\le nu(X)^{κ(X)aL(X)}$. In 1979, A. V. Arhangelskii asked if the inequality $|X|\le 2^{χ(X)wL_c(X)}$ was true for every Hausdorff space $X$. It follows from the third inequality that the answer of this question is in the affirmative for all spaces with $nu(X)$ not greater than the cardinality of the continuum. We also give a simple example of a Hausdorff space $X$ such that $|\mathrm{cl}_θ(A)|>|A|^{χ(X)}U(X)$ and $|\mathrm{cl}_θ(A)|>(|A|\cdot U(X))^{χ(X)}$, where $U(X)$ is the Urysohn number of $X$, recently introduced by Bonanzinga, Cammaroto and Matveev. This example shows that in (1) and (2) above, $nu(X)$ cannot be replaced by $U(X)$ and answers some questions posed by Bella and Cammaroto (1988), Bonanzinga, Cammaroto and Matveev (2011), and Bonanzinga and Pansera (2012).