Lemma 67.52.1. Let S be a scheme. Let f : X \to Y be a morphism of algebraic spaces over S which is representable, of finite type, and separated. Let Y' be the normalization of Y in X. Picture:
\xymatrix{ X \ar[rd]_ f \ar[rr]_{f'} & & Y' \ar[ld]^\nu \\ & Y & }
Then there exists an open subspace U' \subset Y' such that
(f')^{-1}(U') \to U' is an isomorphism, and
(f')^{-1}(U') \subset X is the set of points at which f is quasi-finite.
Proof.
Let W \to Y be a surjective étale morphism where W is a scheme. Then W \times _ Y X is a scheme as well. By Lemma 67.48.4 the algebraic space W \times _ Y Y' is representable and is the normalization of the scheme W in the scheme W \times _ Y X. Picture
\xymatrix{ W \times _ Y X \ar[rd]_{(1, f)} \ar[rr]_{(1, f')} & & W \times _ Y Y' \ar[ld]^{(1, \nu )} \\ & W & }
By More on Morphisms, Lemma 37.43.1 the result of the lemma holds over W. Let V' \subset W \times _ Y Y' be the open subscheme such that
(1, f')^{-1}(V') \to V' is an isomorphism, and
(1, f')^{-1}(V') \subset W \times _ Y X is the set of points at which (1, f) is quasi-finite.
By Lemma 67.34.7 there is a maximal open set of points U \subset X where f is quasi-finite and W \times _ Y U = (1, f')^{-1}(V'). The morphism f'|_ U : U \to Y' is an open immersion by Lemma 67.12.1 as its base change to W is the isomorphism (1, f')^{-1}(V') \to V' followed by the open immersion V' \to W \times _ Y Y'. Setting U' = \mathop{\mathrm{Im}}(U \to Y') finishes the proof (omitted: the verification that (f')^{-1}(U') = U).
\square
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