Lemma 10.158.10. Let $K/k$ be a finitely generated field extension. Then $K$ is separable over $k$ if and only if $K$ is the localization of a smooth $k$-algebra.

Proof. Choose a finite type $k$-algebra $R$ which is a domain whose fraction field is $K$. Lemma 10.140.9 says that $k \to R$ is smooth at $(0)$ if and only if $K/k$ is separable. This proves the lemma. $\square$

There are also:

• 2 comment(s) on Section 10.158: Formal smoothness of fields

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).