Remark 94.7.3. Ignoring set theoretical difficulties^{1} $\mathcal{S}\! \mathit{paces}$ also satisfies descent for objects and hence is a stack. Namely, we have to show that given

an fppf covering $\{ U_ i \to U\} _{i \in I}$,

for each $i \in I$ an algebraic space $X_ i/U_ i$, and

for each $i, j \in I$ an isomorphism $\varphi _{ij} : X_ i \times _ U U_ j \to U_ i \times _ U X_ j$ of algebraic spaces over $U_ i \times _ U U_ j$ satisfying the cocycle condition over $U_ i \times _ U U_ j \times _ U U_ k$,

there exists an algebraic space $X/U$ and isomorphisms $X_{U_ i} \cong X_ i$ over $U_ i$ recovering the isomorphisms $\varphi _{ij}$. First, note that by Sites, Lemma 7.26.4 there exists a sheaf $X$ on $(\mathit{Sch}/U)_{fppf}$ recovering the $X_ i$ and the $\varphi _{ij}$. Then by Bootstrap, Lemma 79.11.1 we see that $X$ is an algebraic space (if we ignore the set theoretic condition of that lemma). We will use this argument in the next section to show that if we consider only algebraic spaces of finite type, then we obtain a stack.

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