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 SOP₂ characterizes maximality in the interpretability order ∇*, settling a prior conjecture and proving that SOP₂ is a real dividing line. Third, we establish the beginnings of a structure theory for NSOP₂, proving that NSOP₂ can be characterized by the existence of few so-called higher formulas. In the course of the paper, we show that p_s = t_s 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.