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arXiv · 2605.12323

Trace definability II: model-theoretic linearity

Erik Walsberg
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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 $\mathscr{M}$ such that $\mathscr{M}$ does not interpret an infinite group but the Shelah completion of $\mathscr{M}$ 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.

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Erik Walsberg. 2026-05-12. Trace definability II: model-theoretic linearity. https://arxiv.org/abs/2605.12323

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