On topological groups admitting a base at identity indexed with $ω^ω$

Abstract

A topological group $G$ is said to have a local $ω^ω$-base if the neighbourhood system at identity admits a monotone cofinal map from the directed set $ω^ω$. In particular, every metrizable group is such, but the class of groups with a local $ω^ω$-base is significantly wider. The aim of this article is to better understand the boundaries of this class, by presenting new examples and counter-examples. Ultraproducts and non-arichimedean ordered fields lead to natural families of non-metrizable groups with a local $ω^ω$-base which nevertheless are Baire topological spaces. More examples come from such constructions as the free topological group $F(X)$ and the free Abelian topological group $A(X)$ of a Tychonoff (more generally uniform) space $X$, as well as the free product of topological groups. We show that 1) the free product of countably many separable topological groups with a local $ω^ω$-base admits a local $ω^ω$-base; 2) the group $A(X)$ of a Tychonoff space $X$ admits a local $ω^ω$-base if and only if the finest uniformity of $X$ has a $ω^ω$-base; 3) the group $F(X)$ of a Tychonoff space $X$ admits a local $ω^ω$-base provided $X$ is separable and the finest uniformity of $X$ has a $ω^ω$-base.