Lemma 76.39.3. Let S be a scheme. Let f : X \to B be a morphism of algebraic spaces over S. Let U \subset B be an open subspace. Assume
B is quasi-compact and quasi-separated,
U is quasi-compact,
f : X \to B is proper, and
f^{-1}(U) \to U is an isomorphism.
Then there exists a U-admissible blowup B' \to B such that the strict transform X' of X maps isomorphically to B'.
Proof.
By Lemma 76.39.1 we may assume that X \to B is flat and of finite presentation. After replacing B by a U-admissible blowup if necessary, we may assume that U \subset B is scheme theoretically dense. Then f is finite by Lemma 76.37.4 and an open immersion by Lemma 76.37.5. Hence f is an open immersion whose image is closed and contains the dense open U, whence f is an isomorphism.
\square
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