Protasov, I.V.
(Algebra and Discrete Mathematics, 2002)
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 ...