$Q$-spaces, perfect spaces and related cardinal characteristics of the continuum
Volume 125 / 2023
Banach Center Publications 125 (2023), 9-15
MSC: Primary 54D10; Secondary 03E15, 03E17, 03E35, 03E50, 54A35, 54H05.
DOI: 10.4064/bc125-1
Abstract
A topological space $X$ is called a $Q$-space if every subset of $X$ is of type $F_\sigma$ in $X$. For $i\in\{1,2,3\}$ let $\mathfrak q_i$ be the smallest cardinality of a second-countable $T_i$-space which is not a $Q$-space. It is clear that $\mathfrak q_1\le\mathfrak q_2\le\mathfrak q_3$. For $i\in\{1,2\}$ we prove that $\mathfrak q_i$ is equal to the smallest cardinality of a second-countable $T_i$-space which is not perfect. Also we prove that $\mathfrak q_3$ is equal to the smallest cardinality of a submetrizable space which is not a $Q$-space. Martin’s Axiom implies that $\mathfrak q_i=\mathfrak c$ for all $i\in\{1,2,3\}$.
