Lemma 10.158.5. Let K/k be an extension of fields. If K is formally smooth over k, then K is a separable extension of k.
Proof. Assume K is formally smooth over k. By Lemma 10.138.9 we see that K \otimes _ k \Omega _{k/\mathbf{Z}} \to \Omega _{K/\mathbf{Z}} is injective. Hence K is separable over k by Lemma 10.158.4. \square
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