Fraisse Structures with SDAP+, Part I: Indivisibility
Abstract
This is Part I of a two-part series regarding Ramsey properties of Fraisse structures satisfying a property called SDAP+, which strengthens the Disjoint Amalgamation Property. We prove that every Fraisse structure in a finite relational language with relation symbols of any finite arity satisfying this property is indivisible. Novelties include a new formulation of coding trees in terms of 1-types over initial segments of the Fraisse structure, and a direct proof of indivisibility which uses the method of forcing to conduct unbounded searches for finite sets. In Part II, we prove that every Fraisse structure in a finite relational language with relation symbols of arity at most two having this property has finite big Ramsey degrees which have a simple characterization. It follows that any such Fraisse structure admits a big Ramsey structure. Part II utilizes a theorem from Part I as a pigeonhole principle for induction arguments. This work offers a streamlined and unifying approach to Ramsey theory on some seemingly disparate classes of Fraisse structures.
Cite
@article{arxiv.2207.06393,
title = {Fraisse Structures with SDAP+, Part I: Indivisibility},
author = {Rebecca Coulson and Natasha Dobrinen and Rehana Patel},
journal= {arXiv preprint arXiv:2207.06393},
year = {2022}
}
Comments
At the request of the referee, our preprint arXiv:2010.02034 has been divided into two papers, Fraisse structures with SDAP+, Parts I and II. The property SDAP+ in the original preprint has been divided into a simpler version, again called SDAP+, and a labeled version called LSDAP+. All results in the preprint 2010.02034 remain the same in Parts I and II