Butterfly Points and Hyperspace Selections

Abstract

If $f$ is a continuous selection for the Vietoris hyperspace $\mathscr{F}(X)$ of the nonempty closed subsets of a space $X$, then the point $f(X)\in X$ is not as arbitrary as it might seem at first glance. In this paper, we will characterise these points by local properties at them. Briefly, we will show that $p=f(X)$ is a strong butterfly point precisely when it has a countable clopen base in $\overline{U}$ for some open set $U\subset X\setminus\{p\}$ with $\overline{U}=U\cup\{p\}$. Moreover, the same is valid when $X$ is totally disconnected at $p=f(X)$ and $p$ is only assumed to be a butterfly point. This gives the complete affirmative solution to a question raised previously by the author. Finally, when $p=f(X)$ lacks the above local base-like property, we will show that $\mathscr{F}(X)$ has a continuous selection $h$ with the stronger property that $h(S)=p$ for every closed $S\subset X$ with $p\in S$.