Hausdorff distance between ultrametric balls

Abstract

Let $(X, d)$ be an ultrametric space and let $d_H$ be the Hausdorff distance on the set $\bar{\mathbf{B}}_X$ of all closed balls in $(X, d)$. Some interconnections between the properties of the spaces $(X, d)$ and $(\bar{\mathbf{B}}_X, d_H)$ are described. It is established that the space $(\bar{\mathbf{B}}_X, d_H)$ has such properties as discreteness, local finiteness, metrical discreteness, completeness, compactness, local compactness if and only if the space $(X, d)$ has these properties. Necessary and sufficient conditions for the separability of the space $(\bar{\mathbf{B}}_X, d_H)$ are also proved.