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BINARY PRIMITIVE HOMOGENEOUS SIMPLE STRUCTURES

Published online by Cambridge University Press:  21 March 2017

VERA KOPONEN
VERA KOPONEN*
Affiliation:
DEPARTMENT OF MATHEMATICS UPPSALA UNIVERSITY BOX 480, 75106 UPPSALA, SWEDENE-mail: [email protected]
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Abstract

Suppose that ${\cal M}$ is countable, binary, primitive, homogeneous, and simple. We prove that the SU-rank of the complete theory of ${\cal M}$ is 1 and hence 1-based. It follows that ${\cal M}$ is a random structure. The conclusion that ${\cal M}$ is a random structure does not hold if the binarity condition is removed, as witnessed by the generic tetrahedron-free 3-hypergraph. However, to show that the generic tetrahedron-free 3-hypergraph is 1-based requires some work (it is known that it has the other properties) since this notion is defined in terms of imaginary elements. This is partly why we also characterize equivalence relations which are definable without parameters in the context of ω-categorical structures with degenerate algebraic closure. Another reason is that such characterizations may be useful in future research about simple (nonbinary) homogeneous structures.

Keywords

03C1003C5003C45model theoryhomogeneous structuresimple theory1-based theoryrandom structure
Type
Articles
Information
The Journal of Symbolic Logic , Volume 82 , Issue 1 , March 2017 , pp. 183 - 207
Copyright
Copyright © The Association for Symbolic Logic 2017 

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