The Index and Core of a Relation. With Applications to the Axiomatics of Relation Algebra

Abstract

We introduce the general notions of an index and a core of a relation. We postulate a limited form of the axiom of choice -- specifically that all partial equivalence relations have an index -- and explore the consequences of adding the axiom to standard axiom systems for point-free reasoning. Examples of the theorems we prove are that a core/index of a difunction is a bijection, and that the so-called ``all or nothing'' axiom used to facilitate pointwise reasoning is derivable from our axiom of choice.

Fundamenta Informaticae, Volume 195, Issue 1-4, Article 2, 2025