Forking extension

In model theory, a forking extension is an extension that is not free whereas a non-forking extension is an extension that is as free as possible. This can be used to extend the notions of linear or algebraic independence to stable theories. These concepts were introduced by S. Shelah.

Definitions

Suppose that A and B are models of some complete ω-stable theory T. If p is a type of A and q is a type of B containing p, then q is called a forking extension of p if its Morley rank is smaller, and a nonforking extension if it has the same Morley rank.

Axioms

Let T be a stable complete theory. The non-forking relation ≤ for types over T is the unique relation that satisfies the following axioms:

  1. If pq then pq. If f is an elementary map then pq if and only if fpfq
  2. If pqr then pr if and only if pq and qr
  3. If p is a type of A and AB then there is some type q of B with pq.
  4. There is a cardinal κ such that if p is a type of A then there is a subset A0 of A of cardinality less than κ so that (p|A0) ≤ p, where | stands for restriction.
  5. For any p there is a cardinal λ such that there are at most λ non-contradictory types q with pq.

References

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