# Abstract elementary class

In model theory, a discipline within mathematical logic, an **abstract elementary class**, or **AEC** for short, is a class of models with a partial order similar to the relation of an elementary substructure of an elementary class in first-order model theory. They were introduced by Saharon Shelah.^{[1]}

## Definition

, for a class of structures in some language , is an AEC if it has the following properties:

- is a partial order on .
- If then is a substructure of .
**Isomorphisms**: is closed under isomorphisms, and if and then**Coherence**: If and then**Tarski–Vaught chain axioms**: If is an ordinal and is a chain (i.e. ), then:**Löwenheim–Skolem axiom**: There exists a cardinal , such that if is a subset of the universe of , then there is in whose universe contains such that and . We let denote the least such and call it the**Löwenheim–Skolem number**of .

Note that we usually do not care about the models of size less than the Löwenheim–Skolem number and often assume that there are none (we will adopt this convention in this article). This is justified since we can always remove all such models from an AEC without influencing its structure above the Löwenheim–Skolem number.

A -embedding is a map for such that and is an isomorphism from onto . If is clear from context, we omit it.

## Examples

The following are examples of abstract elementary classes:^{[2]}

- An Elementary class is the most basic example of an AEC: If
*T*is a first-order theory, then the class of models of*T*together with elementary substructure forms an AEC with Löwenheim–Skolem number*|T|*. - If is a sentence in the infinitary logic , and is a countable fragment containing , then is an AEC with Löwenheim–Skolem number . This can be generalized to other logics, like , or , where expresses "there exists uncountably many".
- If
*T*is a first-order countable superstable theory, the set of -saturated models of*T*, together with elementary substructure, is an AEC with Löwenheim–Skolem number . - Zilber's pseudo-exponential fields form an AEC.

## Common assumptions

AECs are very general objects and one usually make some of the assumptions below when studying them:

- An AEC has
*joint embedding*if any two model can be embedded inside a common model. - An AEC has
*no maximal model*if any model has a proper extension. - An AEC has
*amalgamation*if for any triple with , , there is and embeddings of and inside that fix pointwise.

Note that in elementary classes, joint embedding holds whenever the theory is complete, while amalgamation and no maximal models are well-known consequences of the compactness theorem. These three assumptions allow us to build a universal model-homogeneous monster model , exactly as in the elementary case.

Another assumption that one can make is tameness.

## Shelah's categoricity conjecture

Shelah introduced AECs to provide a uniform framework in which to generalize first-order classification theory. Classification theory started with Morley's categoricity theorem, so it is natural to ask whether a similar result holds in AECs. This is **Shelah's categoricity conjecture**. It states that there should be a Hanf number for categoricity:

For every AEC *K* there should be a cardinal depending only on such that if *K* is categorical in *some* (i.e. *K* has exactly one (up to isomorphism) model of size ), then *K* is categorical in for *all* .

Several approximations have been published (see for example the results section below), assuming set-theoretic assumptions (such as the existence of large cardinals or variations of the generalized continuum hypothesis), or model-theoretic assumptions (such as amalgamation or tameness). As of 2014, the original conjecture remains open.

## Results

The following are some important results about AECs. Except for the last, all results are due to Shelah.

**Shelah's Presentation Theorem**:^{[3]}Any AEC is : it is a reduct of a class of models of a first-order theory omitting at most types.**Hanf number for existence**:^{[4]}Any AEC which has a model of size has models of arbitrarily large sizes.**Amalgamation from categoricity**:^{[5]}If*K*is an AEC categorical in and and , then*K*has amalgamation for models of size .**Existence from categoricity**:^{[6]}If*K*is a AEC with Löwenheim–Skolem number and*K*is categorical in and , then*K*has a model of size . In particular, no sentence of can have exactly one uncountable model.**Approximations to Shelah's categoricity conjecture**:**Downward transfer from a successor**:^{[7]}If*K*is an abstract elementary class with amalgamation that is categorical in a "high-enough" successor , then*K*is categorical in all high-enough .**Shelah's categoricity conjecture for a successor from large cardinals**:^{[8]}If there are class-many strongly compact cardinals, then Shelah's categoricity conjecture holds when we start with categoricity at a successor.

## See also

## Notes

- ↑ Template:Harvnb.
- ↑ Template:Harvnb, Section 1.
- ↑ Template:Harvnb, Theorem 3.4.
- ↑ Template:Harvnb, Corollary 3.5. Note that there is a typo there and that should be replaced by .
- ↑ Template:Harvnb, Theorem 4.3.
- ↑ Template:Harvnb, Theorem 5.1.
- ↑ Template:Harvnb.
- ↑ This is due to Will Boney, but combines results of many people, including Grossberg, Makkai, Shelah, and VanDieren. A proof appears in Template:Harvnb, Theorem 7.5.

## References

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