Lemma 38.32.2. Let S be a quasi-compact and quasi-separated scheme. Let f : X \to Y be a morphism of schemes over S with Y separated and of finite type over S and X compactifyable over S. Then X has a compactification over Y.
Proof. Let j : X \to \overline{X} be a compactification of X over S. Then we let \overline{X}' be the scheme theoretic image of (j, f) : X \to \overline{X} \times _ S Y. The morphism \overline{X}' \to Y is proper because \overline{X} \times _ S Y \to Y is proper as a base change of \overline{X} \to S. On the other hand, since Y is separated over S, the morphism (1, f) : X \to X \times _ S Y is a closed immersion (Schemes, Lemma 26.21.10) and hence X \to \overline{X}' is an open immersion by Morphisms, Lemma 29.6.8 applied to the “partial section” s = (j, f) to the projection \overline{X} \times _ S Y \to \overline{X}. \square
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