Quasi-finite, separated morphisms are quasi-affine
Lemma 37.43.2. Let f : X \to S be a morphism of schemes. Assume f is quasi-finite and separated. Let S' be the normalization of S in X, see Morphisms, Definition 29.53.3. Picture:
\xymatrix{ X \ar[rd]_ f \ar[rr]_{f'} & & S' \ar[ld]^\nu \\ & S & }
Then f' is a quasi-compact open immersion and \nu is integral. In particular f is quasi-affine.
Proof.
This follows from Lemma 37.43.1. Namely, by that lemma there exists an open subscheme U' \subset S' such that (f')^{-1}(U') = X and X \to U' is an isomorphism. In other words, f' is an open immersion. Note that f' is quasi-compact as f is quasi-compact and \nu : S' \to S is separated (Schemes, Lemma 26.21.14). It follows that f is quasi-affine by Morphisms, Lemma 29.13.3.
\square
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