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The Stacks project

Lemma 101.27.14. Let f_ j : \mathcal{X}_ j \to \mathcal{X}, j \in J be a family of morphisms of algebraic stacks which are each flat and locally of finite presentation and which are jointly surjective, i.e., |\mathcal{X}| = \bigcup |f_ j|(|\mathcal{X}_ j|). Then for every scheme U and object x of \mathcal{X} over U there exists an fppf covering \{ U_ i \to U\} _{i \in I}, a map a : I \to J, and objects x_ i of \mathcal{X}_{a(i)} over U_ i such that f_{a(i)}(x_ i) \cong y|_{U_ i} in \mathcal{X}_{U_ i}.

Proof. Apply Lemma 101.27.13 to the morphism \coprod _{j \in J} \mathcal{X}_ j \to \mathcal{X}. (There is a slight set theoretic issue here – due to our setup of things – which we ignore.) To finish, note that a morphism x_ i : U_ i \to \coprod _{j \in J} \mathcal{X}_ j is given by a disjoint union decomposition U_ i = \coprod U_{i, j} and morphisms U_{i, j} \to \mathcal{X}_ j. Then the fppf covering \{ U_{i, j} \to U\} and the morphisms U_{i, j} \to \mathcal{X}_ j do the job. \square


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