Central extensions of preordered groups

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 so-called commutative objects, which are precisely the preordered groups whose group law is commutative. The second one is the category of abelian objects, which 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 positive cones is a special Schreier surjection of monoids.