Pure quotients and Morita's theorem for $k_ω-spaces

Abstract

A $k_{\omega }$-space $X$ is a Hausdorff quotient of a locally compact, $\sigma$-compact Hausdorff space. A theorem of Morita's describes the structure of $X$ when the quotient map is closed, but in 2010 a question of Arkhangel'skii's highlighted the lack of a corresponding theorem for non-closed quotient maps (even from subsets of ${\mathbb{R}}^n$). Arkhangel'skii's specific question had in fact been answered by Siwiec in 1976, but a general structure theorem for $k_{\omega }$-spaces is still lacking. We introduce pure quotient maps, extend Morita's theorem to these, and use Fell's topology to show that every quotient map can be 'purified' (and thus every $k_{\omega }$-space is the image of a pure quotient map). This clarifies the structure of arbitrary $k_{\omega }$-spaces and gives a fuller answer to Arkhangel'skii's question.

To appear in the Canadian Mathematical Bulletin. arXiv admin note: substantial text overlap with arXiv:2010.03741