Keisler's order has infinitely many classes

Abstract

We prove, in ZFC, that there is an infinite strictly descending chain of classes of theories in Keisler's order. Thus Keisler's order is infinite and not a well order. Moreover, this chain occurs within the simple unstable theories, considered model-theoretically tame. Keisler's order is a central notion of the model theory of the 60s and 70s which compares first-order theories (and implicitly ultrafilters) according to saturation of ultrapowers. Prior to this paper, it was long thought to have finitely many classes, linearly ordered. The model-theoretic complexity we find is witnessed by a very natural class of theories, the $n$-free $k$-hypergraphs studied by Hrushovski. This complexity reflects the difficulty of amalgamation and appears orthogonal to forking.

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References in corpus (1)

• Existence of optimal ultrafilters and the fundamental complexity of simple theories

Cited by in corpus (3)

• Low is a Dividing Line in Keisler's Order
• Existence of optimal ultrafilters and the fundamental complexity of simple theories
• On positive local combinatorial dividing-lines in model theory