Forking is Local

Theorem
Let $T$ be a complete $\mathcal{L}$-theory.

Let $\mathfrak{C}$ be a monster model for $T$.

Let $A\subseteq B$ be subsets of the universe of $\mathfrak{C}$.

Let $\pi(\bar x)$ be an $n$-type over $B$.

$\pi$ forks over $A$ if and only if a finite subset of $\pi$ forks over $A$.

Proof
The proof is straightforward using the definition of forking and the fact that proofs in first-order logic are finite.

Suppose $\pi$ forks over $A$.


 * By definition, $\pi$ implies a disjunction of formulas which each divide over $A$.


 * Since proofs are finite, this means that there is a finite subset of $\pi$ which implies this disjunction, completing this direction of the proof.

Suppose a finite subset of $\pi$ forks over $A$.


 * By definition, the finite subset implies a disjunction of formulas which each divide over $A$.


 * But then $\pi$ clearly implies this disjunction as well, completing this direction of the proof.