The Stacks project

Proof. Choose a factorization $W \to Y \to X$ where $j : W \to Y$ is a quasi-compact open immersion and $\pi : Y \to X$ is a finite morphism of finite presentation (Lemma 76.34.4). Let $V = j(W) \cup \pi ^{-1}(U) \subset Y$. Note that $\mathcal{I}$ on $W \cong j(W)$ and $\mathcal{O}_{\pi ^{-1}(U)}$ glue to a finite type quasi-coherent sheaf of ideals $\mathcal{I}_1 \subset \mathcal{O}_ V$. By Limits of Spaces, Lemma 70.9.8 there exists a finite type quasi-coherent sheaf of ideals $\mathcal{I}_2 \subset \mathcal{O}_ Y$ such that $\mathcal{I}_2|_ V = \mathcal{I}_1$. In other words, $\mathcal{I}_2 \subset \mathcal{O}_ Y$ is a finite type quasi-coherent sheaf of ideals such that $V(\mathcal{I}_2)$ is disjoint from $\pi ^{-1}(U)$ and $j^{-1}\mathcal{I}_2 = \mathcal{I}$. Denote $i : Z \to Y$ the corresponding closed immersion which is of finite presentation (Morphisms of Spaces, Lemma 67.28.12). In particular the composition $\tau = \pi \circ i : Z \to X$ is finite and of finite presentation (Morphisms of Spaces, Lemmas 67.28.2 and 67.45.4).

Let $\mathcal{F} = \tau _*\mathcal{O}_ Z$ which we think of as a quasi-coherent $\mathcal{O}_ X$-module. By Descent on Spaces, Lemma 74.6.7 we see that $\mathcal{F}$ is a finitely presented $\mathcal{O}_ X$-module. Let $\mathcal{J} = \text{Fit}_0(\mathcal{F})$. (Insert reference to fitting modules on ringed topoi here.) This is a finite type quasi-coherent sheaf of ideals on $X$ (as $\mathcal{F}$ is of finite presentation, see More on Algebra, Lemma 15.8.4). Part (1) of the lemma holds because $|\tau |(|Z|) \cap |U| = \emptyset $ by our choice of $\mathcal{I}_2$ and because the $0$th Fitting ideal of the trivial module equals the structure sheaf. To prove (2) note that $\varphi ^{-1}(\mathcal{J})\mathcal{O}_ W = \text{Fit}_0(\varphi ^*\mathcal{F})$ because taking Fitting ideals commutes with base change. On the other hand, as $\varphi : W \to X$ is separated and étale we see that $(1, j) : W \to W \times _ X Y$ is an open and closed immersion. Hence $W \times _ Y Z = V(\mathcal{I}) \amalg Z'$ for some finite and finitely presented morphism of algebraic spaces $\tau ' : Z' \to W$. Thus we see that

the second equality by More on Algebra, Lemma 15.8.4 translated in sheaves on ringed topoi. Setting $\mathcal{I}' = \text{Fit}_0(\tau '_*\mathcal{O}_{Z'})$ finishes the proof of the lemma. $\square$

Proof. Choose an affine scheme $V$ and a surjective étale morphism $V \to X$. Because strict transform commutes with étale localization (Divisors on Spaces, Lemma 71.18.2) it suffices to prove the result with $X$ replaced by $V$. Hence we may assume that $X \to B$ is representable (in addition to the hypotheses of the lemma).

Assume that $X \to B$ is representable. Choose an affine scheme $W$ and a surjective étale morphism $\varphi : W \to B$. Note that $X \times _ B W$ is a scheme. By the case of schemes (More on Flatness, Theorem 38.30.7) we can find a finite type quasi-coherent sheaf of ideals $\mathcal{I} \subset \mathcal{O}_ W$ such that (a) $|V(\mathcal{I})| \cap |\varphi ^{-1}(U)| = \emptyset $ and (b) the strict transform of $\mathcal{F}|_{X \times _ B W}$ with respect to the blowing up $W' \to W$ in $\mathcal{I}$ becomes flat over $W'$ and is a module of finite presentation. Choose a finite type sheaf of ideals $\mathcal{J} \subset \mathcal{O}_ B$ as in Lemma 76.38.1. Let $B' \to B$ be the blowing up of $\mathcal{J}$. We claim that this blowup works. Namely, it is clear that $B' \to B$ is $U$-admissible by our choice of ideal $\mathcal{J}$. Moreover, the base change $B' \times _ B W \to W$ is the blowup of $W$ in $\varphi ^{-1}\mathcal{J} = \mathcal{I}\mathcal{I}'$ (compatibility of blowup with flat base change, see Divisors on Spaces, Lemma 71.17.3). Hence there is a factorization

where the first morphism is a blowup as well, see Divisors on Spaces, Lemma 71.17.10). The restriction of $\mathcal{F}'$ (which lives on $B' \times _ B X$) to $W \times _ B B' \times _ B X$ is the strict transform of $\mathcal{F}|_{X \times _ B W}$ (Divisors on Spaces, Lemma 71.18.2) and hence is the twice repeated strict transform of $\mathcal{F}|_{X \times _ B W}$ by the two blowups displayed above (Divisors on Spaces, Lemma 71.18.7). After the first blowup our sheaf is already flat over the base and of finite presentation (by construction). Whence this holds after the second strict transform as well (since this is a pullback by Divisors on Spaces, Lemma 71.18.4). Thus we see that the restriction of $\mathcal{F}'$ to an étale cover of $B' \times _ B X$ has the desired properties and the theorem is proved. $\square$