Dense-codense expansions of quasiminimal pregeometry structures

Abstract

We study expansions of quasiminimal pregeometry structures with a dense codense unary predicate and their relation with the complexity properties of the pregeometry of the underlying structure. We consider beautiful pairs as well as $H$-structures. We show each of these expansions can be axiomatized with a single $L_{ω_1 ω}(Q)$-sentence and that both expansions are $ω$-stable. For $H$-structures we provide a natural notion of independence in the expansion and when the underlying structure is modular, we also provide a natural notion of independence for beautiful pairs. Then we relate the complexity of the pregeometry to properties of the expansions.