English

Countable models of the theories of Baldwin-Shi hypergraphs and their regular types

Logic 2018-07-17 v2

Abstract

We continue the study of the theories of Baldwin-Shi hypergraphs from [5][5]. Restricting our attention to when the rank δ\delta is rational valued, we show that each countable model of the theory of a given Baldwin-Shi hypergraph is isomorphic to a generic structure built from some suitable subclass of the original class of finite structures with the inherited notion of strong substructure. We introduce a notion of dimension for a model and show that there is a an elementary chain {Mβ:β<ω+1}\{\mathfrak{M}_{\beta}:\beta<\omega+1\} of countable models of the theory of a fixed Baldwin-Shi hypergraph with MβMγ\mathfrak{M}_{\beta}\preccurlyeq\mathfrak{M}_\gamma if and only if the dimension of Mβ\mathfrak{M}_\beta is at most the dimension of Mγ\mathfrak{M}_\gamma and that each countable model is isomorphic to some Mβ\mathfrak{M}_\beta. We also study the regular types that appear in these theories and show that the dimension of a model is determined by a particular regular type. Further, drawing on the work of Brody and Laskowski, we use these structures to give an example of a pseudofinite, ω\omega-stable theory with a non-locally modular regular type, answering a question of Pillay in [9][9].

Keywords

Cite

@article{arxiv.1804.00932,
  title  = {Countable models of the theories of Baldwin-Shi hypergraphs and their regular types},
  author = {Danul K. Gunatilleka},
  journal= {arXiv preprint arXiv:1804.00932},
  year   = {2018}
}

Comments

12 pages