| Popis: |
The countably paracompact normal spaces were characterised by Dowker and Kat\v{e}tov in terms of an insertion property. Dowker also characterised them by normality of their product with the closed unit interval. Michael used the Dowker-Kat\v{e}tov insertion property to motivate his selection characterisation of these spaces. Morita extended in a natural way Dowker's product characterisation to all $\tau$-paracompact normal spaces. In this paper, we look at these results from the point of view of open relations. Insertions and selections are equivalent for such relations. Furthermore, we obtain a natural characterisation of $\tau$-paracompact normal spaces in terms of selections for convex-valued open relations. Based on this characterisation, we give simple alternative proofs of the above mentioned results. Other applications are obtained as well. |
