Insertion of Continuous Set-Valued Mappings

TL;DR

This paper characterizes certain topological spaces using set-valued insertion properties, linking them to classical theorems like Dowker's and Katětov-Tong, and extends these concepts to various types of normal spaces.

Contribution

It provides a new characterization of $ au$-paracompact and $ au$-collectionwise normal spaces via set-valued insertion properties, generalizing classical theorems.

Findings

Characterizes $ au$-paracompact normal spaces through set-valued insertion properties.

Shows equivalence of the insertion property to Dowker's theorem for countably paracompact spaces.

Establishes the connection between the insertion property and Katětov-Tong theorem for normal spaces.

Abstract

An interesting result about the existence of "intermediate" set-valued mappings between pairs of such mappings was obtained by Nepomnyashchii. His construction was for a paracompact domain, and he remarked that his result is similar to Dowker's insertion theorem and may represent a generalisation of this theorem. In the present paper, we characterise the $\tau$-paracompact normal spaces by this set-valued "insertion" property and for $\tau=\omega$, i.e. for countably paracompact normal spaces, we show that it is indeed equivalent to the mentioned Dowker's theorem. Moreover, we obtain a similar result for $\tau$-collectionwise normal spaces and show that for normal spaces, i.e. for $\omega$-collectionwise normal spaces, our result is equivalent to the Kat\v{e}tov-Tong insertion theorem. Several related results are obtained as well.