Regular $G_\delta$-diagonals and some upper bounds for cardinality of topological spaces

Abstract

We prove that, under CH, any space with a regular $G_\delta$-diagonal and caliber $\omega_1$ is separable; a corollary of this result answers, under CH, a question of Buzyakova. For any Urysohn space $X$, we establish the inequality $|X|\le wL(X)^{s\Delta_2(X)\cdot{dot(X)}}$ which represents a generalization of a theorem of Basile, Bella, and Ridderbos. We also show that if $X$ is a Hausdorff space, then $|X|\le(\pi\chi(X)\cdot d(X))^{ot(X)\cdot\psi_c(X)}$; this result implies \v{S}apirovski{\u\i}'s inequality $|X|\le\pi\chi(X)^{c(X)\cdot\psi(X)}$ which only holds for regular spaces. It is also proved that $|X|\le \pi\chi(X)^{ot(X)\cdot\psi_c(X)\cdot aL_c(X)}$ for any Hausdorff space $X$; this gives one more generalization of the famous Arhangel$^\prime$skii's inequality $|X|\le 2^{\chi(X)\cdot L(X)}$.