Theorem 13.3. Assuming that:

Then there exists a field

K_{0}

K \subseteq K_{0} \subseteq L

and such that

Moreover $[L : K_{0}] = e_{L ∕ K}$ , $[K_{0} : K] = f_{L ∕ K}$ and $K_{0} ∕ K$ is Galois.