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INDEPENDENCE IN GENERIC INCIDENCE STRUCTURES

Published online by Cambridge University Press:  15 February 2019

GABRIEL CONANT  and
ALEX KRUCKMAN
GABRIEL CONANT
Affiliation:
DEPARTMENT OF MATHEMATICS UNIVERSITY OF NOTRE DAME NOTRE DAME, IN46556, USAE-mail: gconant@nd.edu
ALEX KRUCKMAN
Affiliation:
DEPARTMENT OF MATHEMATICS INDIANA UNIVERSITY BLOOMINGTON BLOOMINGTON, IN47405, USAE-mail: akruckma@indiana.edu
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Abstract

We study the theory Tm,n of existentially closed incidence structures omitting the complete incidence structure Km,n, which can also be viewed as existentially closed Km,n-free bipartite graphs. In the case m = n = 2, this is the theory of existentially closed projective planes. We give an $\forall \exists$-axiomatization of Tm,n, show that Tm,n does not have a countable saturated model when m, n ≥ 2, and show that the existence of a prime model for T2,2 is equivalent to a longstanding open question about finite projective planes. Finally, we analyze model theoretic notions of complexity for Tm,n. We show that Tm,n is NSOP1, but not simple when m, n ≥ 2, and we show that Tm,n has weak elimination of imaginaries but not full elimination of imaginaries. These results rely on combinatorial characterizations of various notions of independence, including algebraic independence, Kim independence, and forking independence.

Keywords

Primary 03C1003C4503C5251E1551E30Secondary 03C6551E30projective planesbipartite graphsNSOP1independence relationsforking
Type
Articles
Information
The Journal of Symbolic Logic , Volume 84 , Issue 2 , June 2019 , pp. 750 - 780
Copyright
Copyright © The Association for Symbolic Logic 2019 

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