$Q$-spaces, perfect spaces and related cardinal characteristics of the continuum

TL;DR

This paper investigates the cardinal characteristics of the continuum related to $Q$-spaces, establishing equalities with smallest non-perfect spaces and analyzing implications under Martin's Axiom.

Contribution

It characterizes the smallest cardinalities of certain second-countable spaces that are not $Q$-spaces and relates these to perfect and submetrizable spaces.

Findings

$rak q_1$, $rak q_2$, $rak q_3$ are equal to smallest non-perfect space cardinalities

Under Martin's Axiom, $rak q_i$ equals the continuum for all $i$

The paper links $Q$-spaces with perfect and submetrizable spaces

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 $i\in\{1,2,3\}$.