Lüroth's theorem

In mathematics, Lüroth's theorem asserts that every field that lies between two other fields K and K(X) must be generated as an extension of K by a single element of K(X). This result is named after Jacob Lüroth, who proved it in 1876.

Statement

Let $K$ be a field and $M$ be an intermediate field between $K$ and $K(X)$, for some indeterminate X. Then there exists a rational function $f(X)\in K(X)$ such that $M=K(f(X))$. In other words, every intermediate extension between $K$ and $K(X)$ is a simple extension.

Proofs

The proof of Lüroth's theorem can be derived easily from the theory of rational curves, using the geometric genus. This method is non-elementary, but several short proofs using only the basics of field theory have long been known. Many of these simple proofs use Gauss's lemma on primitive polynomials as a main step.