Two Weak Forms of Countability Axioms in Free Topological Groups

TL;DR

This paper investigates two weak countability properties, $csf$-countability and $snf$-countability, in free topological groups and their subspaces, providing characterizations and generalizations of previous results.

Contribution

It introduces characterizations of $csf$- and $snf$-countability in free topological groups for various spaces, extending earlier work and answering an open question.

Findings

Characterizations of $csf$-countability and $snf$-countability for different classes of spaces.

Generalizations of results by Arhangel'skind Yamada.

Resolution of an open question by Li et al.

Abstract

Given a Tychonoff space $X$, let $F(X)$ and $A(X)$ be respectively the free topological group and the free Abelian topological group over $X$ in the sense of Markov. For every $n\in\mathbb{N}$, let $F_{n}(X)$ (resp. $A_n(X)$) denote the subspace of $F(X)$ (resp. $A(X)$) that consists of words of reduced length at most $n$ with respect to the free basis $X$. In this paper, we discuss two weak forms of countability axioms in $F(X)$ or $A(X)$, namely the $csf$-countability and $snf$-countability. We provide some characterizations of the $csf$-countability and $snf$-countability of $F(X)$ and $A(X)$ for various classes of spaces $X$. In addition, we also study the $csf$-countability and $snf$-countability of $F_n(X)$ or $A_n(X)$, for $n=2, 3, 4$. Some results of Arhangel'ski\v\i\ in \cite{A1980} and Yamada in \cite{Y1998} are generalized. An affirmative answer to an open question posed by Li et al. in \cite{LLL} is provided.