Companionability Characterization for the Expansion of an O-minimal Theory by a Dense Subgroup.

This paper provides a full characterization for when the expansion of a complete o-minimal theory by a unary predicate that picks out a dense and divisible proper subgroup has a model companion. This result is motivated by criteria and questions introduced in recent works concerning the existence of model companions, as well as preservation results for some neostability properties. The focus of this paper is establishing the companionability dividing line in the o-minimal setting because this allows us to provide a full and geometric characterization. Examples are included both in which the predicate is an additive subgroup, and where it is a mutliplicative subgroup. The paper concludes with a brief discussion of neostability properties and examples that illustrate the lack of preservation for properties such as strong, NIP, and NTP$_2$, though there are also examples for which some or all three of those properties hold.