

Let <![CDATA[\mathcal M=(M, be a linearly ordered first-order structure and T its complete theory. We investigate conditions for T that could guarantee that is not much more complex than some colored orders (linear orders with added unary predicates). Motivated by Rubin's work [5], we label three conditions expressing properties of types of T and/or automorphisms of models of T. We prove several results which indicate the geometric simplicity of definable sets in models of theories satisfying these conditions. For example, we prove that the strongest condition characterizes, up to definitional equivalence (inter-definability), theories of colored orders expanded by equivalence relations with convex classes. Copyright © 2021 by the Association for Symbolic Logic. All rights reserved.
| Funding sponsor | Funding number | Acronym |
|---|---|---|
| Univerzitet u Beogradu | ||
| Ministarstvo Prosvete, Nauke i Tehnološkog Razvoja | MPNTR | |
| Narodowe Centrum Nauki | 2016/22/E/ST1/00450 | NCN |
Proof of Theorem 1. It suffices to show that a saturated linearly ordered structure is definitionally equivalent with its ccel-reduct if and only if it satisfies (SLB). The right-to-left direction is a consequence of Theorem 2: (SLB) implies that every formula is equivalent with a Boolean combination of u-convex formulae which are expressible in the language of the ccel-reduct, too. The other direction is an immediate consequence of Lemma 2.6. \u22A3 Acknowledgments. The first author was supported by the Ministry of Education, Science and Technological Development of Serbia through Mathematical Institute SANU. The second author was supported by the Narodowe Centrum Nauki grant no. 2016/22/E/ST1/00450, and by the Ministry of Education, Science and Technological Development of Serbia through University of Belgrade, Faculty of Mathematics. The third author was supported by the Ministry of Education, Science and Technological Development of Serbia through University of Belgrade, Faculty of Transport and Traffic Engineering.
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