We propose the notion of a quasiminimal abstract elementary class. This is an AEC satisfying four semantic conditions: countable Löwenheim–Skolem–Tarski number, existence of a prime model, closure under intersections, and uniqueness of the generic orbital type over every countable model. We exhibit a correspondence between Zilber’s quasiminimal pregeometry classes and quasiminimal AECs: any quasiminimal pregeometry class induces a quasiminimal AEC, and for any quasiminimal AEC there is a natural functorial expansion that induces a quasiminimal pregeometry class. We show in particular (...) that the exchange axiom is redundant in Zilber’s definition of a quasiminimal pregeometry class. (shrink)

We initiate a systematic investigation of the abstract elementary classes that have amalgamation, satisfy tameness, and are stable in some cardinal. Assuming the singular cardinal hypothesis, we prove a full characterization of the stability cardinals, and connect the stability spectrum with the behavior of saturated models. We deduce that if a class is stable on a tail of cardinals, then it has no long splitting chains. This indicates that there is a clear notion of superstability in this framework. We also (...) present an application to homogeneous model theory: for D a homogeneous diagram in a first-order theory T, if D is both stable in |T| and categorical in |T| then D is stable in all λ ≥|T|. (shrink)

We present a new proof of the existence of Morley sequences in simple theories. We avoid using the Erdős–Rado theorem and instead use only Ramsey’s theorem and compactness. The proof shows that the basic theory of forking in simple theories can be developed using only principles from “ordinary mathematics,” answering a question of Grossberg, Iovino, and Lessmann, as well as a question of Baldwin.

For a fixed natural number \, the Hart–Shelah example is an abstract elementary class with amalgamation that is categorical exactly in the infinite cardinals less than or equal to \. We investigate recently-isolated properties of AECs in the setting of this example. We isolate the exact amount of type-shortness holding in the example and show that it has a type-full good \-frame which fails the existence property for uniqueness triples. This gives the first example of such a frame. Along the (...) way, we develop new tools to build and analyze good frames. (shrink)

We show how to build prime models in classes of saturated models of abstract elementary classes having a well-behaved independence relation: Let math formula be an almost fully good AEC that is categorical in math formula and has the math formula-existence property for domination triples. For any math formula, the class of Galois saturated models of math formula of size λ has prime models over every set of the form math formula. This generalizes an argument of Shelah, who proved the (...) result when λ is a successor cardinal. (shrink)