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Geometric Stability Theory and the Classification of Unstable Structures
Abstract
The equivalence of NSOP1 and NSOP3, two model-theoretic complexity properties, remains open, and both the classes NSOP1 and NSOP3 are more complex than even the simple unstable theories. And yet, it turns out that classical geometric stability theory, in particular the group configuration theorem of Hrushovski (1992), is capable of controlling classification theory on either side of the NSOP1-SOP3 dichotomy, via the expansion of stable theories by generic predicates and equivalence relations. This allows us to construct new examples of strictly NSOP1 theories. We introduce generic expansions corresponding, though universal axioms, to definable relations in the underlying theory, and discuss the existence of model companions for some of these expansions. In the case where the defining relation in the underlying theory T is a ternary relation R(x,y,z) coming from a surface in 3-space, we give a surprising application of the group configuration theorem to classifying the corresponding generic expansion TR. Namely, when T is weakly minimal and eliminates the quantifier ∃∞, TR is strictly NSOP4 and TP2 exactly when R comes from the graph of a type-definable group operation; otherwise, depending on whether the expansion is by a generic predicate or a generic equivalence relation, it is simple or NSOP1.