Fraïssé limit


In mathematical logic, specifically in the discipline of model theory, the Fraïssé limit is a method used to construct mathematical structures from their substructures. It is a special example of the more general concept of a direct limit in a category. The technique was developed in the 1950s by its namesake, French logician Roland Fraïssé.
The main point of Fraïssé's construction is to show how one can approximate a structure by its finitely generated substructures. Given a class of finite relational structures, if satisfies certain properties, then there exists a unique countable structure, called the Fraïssé limit of, which contains all the elements of as substructures.
The general study of Fraïssé limits and related notions is sometimes called Fraïssé theory. This field has seen wide applications to other parts of mathematics, including topological dynamics, functional analysis, and Ramsey theory.

Finitely generated substructures and age

Fix a language. By an -structure, we mean a logical structure having signature.
Given an -structure with domain, and a subset, we use to denote the least substructure of whose domain contains .
A substructure of is then said to be finitely generated if for some finite subset. The age of, denoted, is the class of all finitely generated substructures of .
One can prove that any class that is the age of some structure satisfies the following two conditions:
Hereditary property
Joint embedding property

Fraïssé's theorem

As above, we noted that for any -structure , satisfies the HP and JEP. Fraïssé proved a sort-of-converse result: when is any non-empty, countable set of finitely generated -structures that has the above two properties, then it is the age of some countable structure.
Furthermore, suppose that happens to satisfy the following additional properties.
Amalgamation property
Essential countability
In that case, we say that K is a Fraïssé class, and there is a unique, countable, homogeneous structure whose age is exactly. This structure is called the Fraïssé limit of.
Here, homogeneous means that any isomorphism between two finitely generated substructures can be extended to an automorphism of the whole structure.

Examples

The archetypal example is the class of all finite linear orderings, for which the Fraïssé limit is a dense linear order without endpoints. Up to isomorphism, this is always equivalent to the structure, i.e. the rational numbers with the usual ordering.
As a non-example, note that neither nor are the Fraïssé limit of. This is because, although both of them are countable and have as their age, neither one is homogeneous. To see this, consider the substructures and, and the isomorphism between them. This cannot be extended to an automorphism of or, since there is no element to which we could map, while still preserving the order.
Another example is the class of all finite graphs, whose Fraïssé limit is the Rado graph.

ω-categoricity

Suppose our class under consideration satisfies the additional property of being uniformly locally finite, which means that for every, there is a uniform bound on the size of an -generated substructure. This condition is equivalent to the Fraïssé limit of being ω-categorical.
For example, the class of finite dimensional vector spaces over a fixed field is always a Fraïssé class, but it is uniformly locally finite only if the field is finite.