Lemma 72.16.2 (07W4)—The Stacks project

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Table of contents
Part 4: Algebraic Spaces
Chapter 72: Algebraic Spaces over Fields
Section 72.16: Spaces smooth over fields
Lemma 72.16.2 (cite)

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Lemma 72.16.2. Let $k$ be a field. Let $X$ be an algebraic space smooth over $\mathop{\mathrm{Spec}}(k)$ . The set of $x \in |X|$ which are image of morphisms $\mathop{\mathrm{Spec}}(k') \to X$ with $k' \supset k$ finite separable is dense in $|X|$ .

Proof. Choose a scheme $U$ and a surjective étale morphism $U \to X$ . The morphism $U \to \mathop{\mathrm{Spec}}(k)$ is smooth as a composition of an étale (hence smooth) morphism and a smooth morphism (see Morphisms of Spaces, Lemmas 67.39.6 and 67.37.2). Hence we can apply Varieties, Lemma 33.25.6 to see that the closed points of $U$ whose residue fields are finite separable over $k$ are dense. This implies the lemma by our definition of the topology on $|X|$ . $\square$

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