Israel Journal of Mathematics

, Volume 220, Issue 2, pp 947–1014 | Cite as

Model-theoretic applications of cofinality spectrum problems



We apply the recently developed technology of cofinality spectrum problems to prove a range of theorems in model theory. First, we prove that any model of Peano arithmetic is λ-saturated iff it has cofinality ≥ λ and the underlying order has no (κ, κ)-gaps for regular κ < λ. We also answer a question about balanced pairs of models of PA. Second, assuming instances of GCH, we prove that SOP 2 characterizes maximality in the interpretability order ∇*, settling a prior conjecture and proving that SOP 2 is a real dividing line. Third, we establish the beginnings of a structure theory for NSOP 2, proving that NSOP 2 can be characterized by the existence of few so-called higher formulas. In the course of the paper, we show that ps = ts in any weak cofinality spectrum problem closed under exponentiation (naturally defined). We also prove that the local versions of these cardinals need not coincide, even in cofinality spectrum problems arising from Peano arithmetic.


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© Hebrew University of Jerusalem 2017

Authors and Affiliations

  1. 1.Department of MathematicsUniversity of ChicagoChicagoUSA
  2. 2.Mathematical Sciences Research InstituteBerkeleyUSA
  3. 3.Einstein Institute of Mathematics, Edmond J. Safra Campus, Givat RamThe Hebrew University of JerusalemJerusalemIsrael
  4. 4.Department of Mathematics, Hill Center - Busch Campus RutgersThe State University of New JerseyPiscatawayUSA

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