In this section we discuss the notion of an fppf covering of algebraic spaces, and we define the big fppf site of an algebraic space. Please compare with Topologies, Section 34.7.
Definition 73.7.1. Let $S$ be a scheme, and let $X$ be an algebraic space over $S$. An fppf covering of $X$ is a family of morphisms $\{ f_ i : X_ i \to X\} _{i \in I}$ of algebraic spaces over $S$ such that each $f_ i$ is flat and locally of finite presentation and such that i.e., the morphisms are jointly surjective.
\[ |X| = \bigcup \nolimits _{i \in I} |f_ i|(|X_ i|), \]
This is exactly the same as Topologies, Definition 34.7.1. In particular, if $X$ and all the $X_ i$ are schemes, then we recover the usual notion of an fppf covering of schemes.
Lemma 73.7.2. Any syntomic covering is an fppf covering, and a fortiori, any smooth, étale, or Zariski covering is an fppf covering.
Proof. This is clear from the definitions, the fact that a syntomic morphism is flat and locally of finite presentation (Morphisms of Spaces, Lemmas 67.36.5 and 67.36.6) and Lemma 73.6.2. $\square$
Lemma 73.7.3. Let $S$ be a scheme. Let $X$ be an algebraic space over $S$.
If $X' \to X$ is an isomorphism then $\{ X' \to X\} $ is an fppf covering of $X$.
If $\{ X_ i \to X\} _{i\in I}$ is an fppf covering and for each $i$ we have an fppf covering $\{ X_{ij} \to X_ i\} _{j\in J_ i}$, then $\{ X_{ij} \to X\} _{i \in I, j\in J_ i}$ is an fppf covering.
If $\{ X_ i \to X\} _{i\in I}$ is an fppf covering and $X' \to X$ is a morphism of algebraic spaces then $\{ X' \times _ X X_ i \to X'\} _{i\in I}$ is an fppf covering.
Proof. Omitted. $\square$
Lemma 73.7.4. Let $S$ be a scheme, and let $X$ be an algebraic space over $S$. Suppose that $\mathcal{U} = \{ f_ i : X_ i \to X\} _{i \in I}$ is an fppf covering of $X$. Then there exists a refinement $\mathcal{V} = \{ g_ i : T_ i \to X\} $ of $\mathcal{U}$ which is an fppf covering such that each $T_ i$ is a scheme.
Proof. Omitted. Hint: For each $i$ choose a scheme $T_ i$ and a surjective étale morphism $T_ i \to X_ i$. Then check that $\{ T_ i \to X\} $ is an fppf covering. $\square$
Lemma 73.7.5. Let $S$ be a scheme. Let $\{ f_ i : X_ i \to X\} _{i \in I}$ be an fppf covering of algebraic spaces over $S$. Then the map of sheaves is surjective.
\[ \coprod X_ i \longrightarrow X \]
Proof. This follows from Spaces, Lemma 65.5.9. See also Spaces, Remark 65.5.2 in case you are confused about the meaning of this lemma. $\square$
Definition 73.7.6. Let $S$ be a scheme. A big fppf site $(\textit{Spaces}/S)_{fppf}$ is any site constructed as follows:
Choose a big fppf site $(\mathit{Sch}/S)_{fppf}$ as in Topologies, Section 34.7.
As underlying category take the category $\textit{Spaces}/S$ of algebraic spaces over $S$ (see discussion in Section 73.2 why this is a set).
Choose any set of coverings as in Sets, Lemma 3.11.1 starting with the category $\textit{Spaces}/S$ and the class of fppf coverings of Definition 73.7.1.
Having defined this, we can localize to get the fppf site of an algebraic space.
Definition 73.7.7. Let $S$ be a scheme. Let $(\textit{Spaces}/S)_{fppf}$ be as in Definition 73.7.6. Let $X$ be an algebraic space over $S$, i.e., an object of $(\textit{Spaces}/S)_{fppf}$. Then the big fppf site $(\textit{Spaces}/X)_{fppf}$ of $X$ is the localization of the site $(\textit{Spaces}/S)_{fppf}$ at $X$ introduced in Sites, Section 7.25.
Next, we establish some relationships between the topoi associated to these sites.
Lemma 73.7.8. Let $S$ be a scheme. Let $f : Y \to X$ be a morphism of algebraic spaces over $S$. The functor is cocontinuous, and has a continuous right adjoint They induce the same morphism of topoi We have $f_{big}^{-1}(\mathcal{G})(U/Y) = \mathcal{G}(U/X)$. We have $f_{big, *}(\mathcal{F})(U/X) = \mathcal{F}(U \times _ X Y/Y)$. Also, $f_{big}^{-1}$ has a left adjoint $f_{big!}$ which commutes with fibre products and equalizers.
\[ u : (\textit{Spaces}/Y)_{fppf} \longrightarrow (\textit{Spaces}/X)_{fppf}, \quad V/Y \longmapsto V/X \]
\[ v : (\textit{Spaces}/X)_{fppf} \longrightarrow (\textit{Spaces}/Y)_{fppf}, \quad (U \to Y) \longmapsto (U \times _ X Y \to Y). \]
\[ f_{big} : \mathop{\mathit{Sh}}\nolimits ((\textit{Spaces}/Y)_{fppf}) \longrightarrow \mathop{\mathit{Sh}}\nolimits ((\textit{Spaces}/X)_{fppf}) \]
Proof. The functor $u$ is cocontinuous, continuous, and commutes with fibre products and equalizers. Hence Sites, Lemmas 7.21.5 and 7.21.6 apply and we deduce the formula for $f_{big}^{-1}$ and the existence of $f_{big!}$. Moreover, the functor $v$ is a right adjoint because given $U/T$ and $V/X$ we have $\mathop{\mathrm{Mor}}\nolimits _ X(u(U), V) = \mathop{\mathrm{Mor}}\nolimits _ Y(U, V \times _ X Y)$ as desired. Thus we may apply Sites, Lemmas 7.22.1 and 7.22.2 to get the formula for $f_{big, *}$. $\square$
Lemma 73.7.9. Let $S$ be a scheme. Given morphisms $f : X \to Y$, $g : Y \to Z$ of algebraic spaces over $S$ we have $g_{big} \circ f_{big} = (g \circ f)_{big}$.
Proof. This follows from the simple description of pushforward and pullback for the functors on the big sites from Lemma 73.7.8. $\square$
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