Lemma 97.4.1. Let $S$ be a scheme. Let

\[ \xymatrix{ X \ar[r] \ar[d] & X' \ar[d] \\ Y \ar[r] & Y' } \]

be a pushout in the category of schemes over $S$ where $X \to X'$ is a thickening and $X \to Y$ is affine, see More on Morphisms, Lemma 37.14.3. Let $\mathcal{Z}$ be an algebraic stack over $S$. Then the functor of fibre categories

\[ \mathcal{Z}_{Y'} \longrightarrow \mathcal{Z}_ Y \times _{\mathcal{Z}_ X} \mathcal{Z}_{X'} \]

is an equivalence of categories.

**Proof.**
Let $y'$ be an object of left hand side. The sheaf $\mathit{Isom}(y', y')$ on the category of schemes over $Y'$ is representable by an algebraic space $I$ over $Y'$, see Algebraic Stacks, Lemma 93.10.11. We conclude that the functor of the lemma is fully faithful as $Y'$ is the pushout in the category of algebraic spaces as well as the category of schemes, see Pushouts of Spaces, Lemma 80.6.1.

Let $(y, x', f)$ be an object of the right hand side. Here $f : y|_ X \to x'|_ X$ is an isomorphism. To finish the proof we have to construct an object $y'$ of $\mathcal{Z}_{Y'}$ whose restrictions to $Y$ and $X'$ agree with $y$ and $x'$ in a manner compatible with $f$. In fact, it suffices to construct $y'$ fppf locally on $Y'$, see Stacks, Lemma 8.4.8. Choose a representable algebraic stack $\mathcal{W}$ and a surjective smooth morphism $\mathcal{W} \to \mathcal{Z}$. Then

\[ (\mathit{Sch}/Y)_{fppf} \times _{y, \mathcal{Z}} \mathcal{W} \quad \text{and}\quad (\mathit{Sch}/X')_{fppf} \times _{x', \mathcal{Z}} \mathcal{W} \]

are algebraic stacks representable by algebraic spaces $V$ and $U'$ smooth over $Y$ and $X'$. The isomorphism $f$ induces an isomorphism $\varphi : V \times _ Y X \to U' \times _{X'} X$ over $X$. By Pushouts of Spaces, Lemmas 80.6.2 and 80.6.7 we see that the pushout $V' = V \amalg _{V \times _ Y X} U'$ is an algebraic space smooth over $Y'$ whose base change to $Y$ and $X'$ recovers $V$ and $U'$ in a manner compatible with $\varphi $.

Let $W$ be the algebraic space representing $\mathcal{W}$. The projections $V \to W$ and $U' \to W$ agree as morphisms over $V \times _ Y X \cong U' \times _{X'} X$ hence the universal property of the pushout determines a morphism of algebraic spaces $V' \to W$. Choose a scheme $Y_1'$ and a surjective étale morphism $Y_1' \to V'$. Set $Y_1 = Y \times _{Y'} Y_1'$, $X_1' = X' \times _{Y'} Y_1'$, $X_1 = X \times _{Y'} Y_1'$. The composition

\[ (\mathit{Sch}/Y_1') \to (\mathit{Sch}/V') \to (\mathit{Sch}/W) = \mathcal{W} \to \mathcal{Z} \]

corresponds by the $2$-Yoneda lemma to an object $y_1'$ of $\mathcal{Z}$ over $Y_1'$ whose restriction to $Y_1$ and $X_1'$ agrees with $y|_{Y_1}$ and $x'|_{X_1'}$ in a manner compatible with $f|_{X_1}$. Thus we have constructed our desired object smooth locally over $Y'$ and we win.
$\square$

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