Trace definability II: model-theoretic linearity

Abstract

We give examples of $\mathrm{NIP}$ structures in which new algebraic structure appears in the Shelah completion. In particular we construct a weakly o-minimal structure $\mathcal{M}$ such that $\mathcal{M}$ does not interpret an infinite group but the Shelah completion of $\mathcal{M}$ interprets an infinite field. We introduce a weak notion of interpretability called local trace definability between first order structures and an associated weak notion of equivalence. We give a dichotomy between ``linearity" and ``field structure" for dp-minimal expansions of archimedean ordered abelian groups. We also prove several other results about trace definability and local trace definability between various classes of structures.

This is the second in a series of papers consisting of cleaned up and strengthened versions of parts of arXiv:2504.05566v1