English

Simple Homogeneous Structures and Indiscernible Sequence Invariants

Logic 2026-04-29 v3

Abstract

We introduce some properties describing dependence in indiscernible sequences: FindF_{ind} and its dual FMbF_{Mb}, the definable Morley property, and nn-resolvability. Applying these properties, we establish the following results: We show that the degree of nonminimality introduced by Freitag and Moosa, which is closely related to FindF_{ind} (equal in DCF0\mathrm{DCF}_{0}), may take on any positive integer value in an ω\omega-stable theory, answering a question of Freitag, Jaoui, and Moosa. Proving a conjecture of Koponen, we show that every simple theory with quantifier elimination in a finite relational language has finite rank and is one-based. The arguments closely rely on finding types qq with FMb(q)=F_{Mb}(q) = \infty, and on nn-resolvability. We prove some variants of the simple Kim-forking conjecture, a generalization of the stable forking conjecture to NSOP1\mathrm{NSOP}_{1} theories. We show a global analogue of the simple Kim-forking conjecture with infinitely many variables holds in every NSOP1\mathrm{NSOP}_{1} theory, and show that Kim-forking with a realization of a type pp with FMb(p)<\mathrm{F}_{Mb}(p) < \infty satisfies a finite-variable version of this result. We then show, in a low NSOP1\mathrm{NSOP}_{1} theory or when pp is isolated, if pS(C)p \in S(C) has the definable Morley property for Kim-independence, Kim-forking with realizations of pp gives a nontrivial instance of the simple Kim-forking conjecture itself. In particular, when FMb(p)<F_{Mb}(p) < \infty and SFMb(p)+1(C)<|S^{F_{Mb}(p) + 1}(C)| < \infty, Kim-forking with realizations of pp gives us a nontrivial instance of the simple Kim-forking conjecture. We show that the quantity FMbF_{Mb}, motivated in simple and NSOP1\mathrm{NSOP}_{1} theories by the above results, is in fact nontrivial even in stable theories.

Cite

@article{arxiv.2405.08211,
  title  = {Simple Homogeneous Structures and Indiscernible Sequence Invariants},
  author = {John Baldwin and James Freitag and Scott Mutchnik},
  journal= {arXiv preprint arXiv:2405.08211},
  year   = {2026}
}

Comments

63 pages

R2 v1 2026-06-28T16:26:08.324Z