Metrizable ball structures

Abstract

A ball structure is a triple (X, P, B), where X, P are nonempty sets and, for any x ∈ X, α ∈ P, B(x, α) is a subset of X, x ∈ B(x, α), which is called a ball of radius α around x. We characterize up to isomorphism the ball structures related to the metric spaces of different types and groups.