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The Stacks project

Lemma 65.15.1. Suppose given big sites \mathit{Sch}_{fppf} and \mathit{Sch}'_{fppf}. Assume that \mathit{Sch}_{fppf} is contained in \mathit{Sch}'_{fppf}, see Topologies, Section 34.12. Let S be an object of \mathit{Sch}_{fppf}. Let

\begin{align*} g : \mathop{\mathit{Sh}}\nolimits ((\mathit{Sch}/S)_{fppf}) \longrightarrow \mathop{\mathit{Sh}}\nolimits ((\mathit{Sch}'/S)_{fppf}), \\ f : \mathop{\mathit{Sh}}\nolimits ((\mathit{Sch}'/S)_{fppf}) \longrightarrow \mathop{\mathit{Sh}}\nolimits ((\mathit{Sch}/S)_{fppf}) \end{align*}

be the morphisms of topoi of Topologies, Lemma 34.12.2. Let F be a sheaf of sets on (\mathit{Sch}/S)_{fppf}. Then

  1. if F is representable by a scheme X \in \mathop{\mathrm{Ob}}\nolimits ((\mathit{Sch}/S)_{fppf}) over S, then f^{-1}F is representable too, in fact it is representable by the same scheme X, now viewed as an object of (\mathit{Sch}'/S)_{fppf}, and

  2. if F is an algebraic space over S, then f^{-1}F is an algebraic space over S also.

Proof. Let X \in \mathop{\mathrm{Ob}}\nolimits ((\mathit{Sch}/S)_{fppf}). Let us write h_ X for the representable sheaf on (\mathit{Sch}/S)_{fppf} associated to X, and h'_ X for the representable sheaf on (\mathit{Sch}'/S)_{fppf} associated to X. By the description of f^{-1} in Topologies, Section 34.12 we see that f^{-1}h_ X = h'_ X. This proves (1).

Next, suppose that F is an algebraic space over S. By Lemma 65.9.1 this means that F = h_ U/h_ R for some étale equivalence relation R \to U \times _ S U in (\mathit{Sch}/S)_{fppf}. Since f^{-1} is an exact functor we conclude that f^{-1}F = h'_ U/h'_ R. Hence f^{-1}F is an algebraic space over S by Theorem 65.10.5. \square


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