Insertion of Continuous Set-Valued Mappings

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.