Central extensions of preordered groups

Abstract

We prove that the category of preordered groups contains two full reflective subcategories that give rise to some interesting Galois theories. The first one is the category of the so-called commutative objects, which are precisely the preordered groups whose group law is commutative. The second one is the category of abelian objects, that turns out to be the category of monomorphisms in the category of abelian groups. We give a precise description of the reflector to this subcategory, and we prove that it induces an admissible Galois structure and then a natural notion of categorical central extension. We then characterize the central extensions of preordered groups in purely algebraic terms: these are shown to be the central extensions of groups having the additional property that their restriction to the positive cones is a special homogeneous surjection of monoids.