Remark 34.11.1. Take any category $\mathit{Sch}_\alpha $ constructed as in Sets, Lemma 3.9.2 starting with the set of schemes $\{ X, Y, S\} $. Choose any set of coverings $\text{Cov}_{fppf}$ on $\mathit{Sch}_\alpha $ as in Sets, Lemma 3.11.1 starting with the category $\mathit{Sch}_\alpha $ and the class of fppf coverings. Let $\mathit{Sch}_{fppf}$ denote the big fppf site so obtained. Next, for $\tau \in \{ Zariski, {\acute{e}tale}, smooth, syntomic\} $ let $\mathit{Sch}_\tau $ have the same underlying category as $\mathit{Sch}_{fppf}$ with coverings $\text{Cov}_\tau \subset \text{Cov}_{fppf}$ simply the subset of $\tau $-coverings. It is straightforward to check that this gives rise to a big site $\mathit{Sch}_\tau $.
34.11 Change of topologies
Let $f : X \to Y$ be a morphism of schemes over a base scheme $S$. In this case we have the following morphisms of sites1 (with suitable choices of sites as in Remark 34.11.1 below):
$(\mathit{Sch}/X)_{fppf} \longrightarrow (\mathit{Sch}/Y)_{fppf}$,
$(\mathit{Sch}/X)_{fppf} \longrightarrow (\mathit{Sch}/Y)_{syntomic}$,
$(\mathit{Sch}/X)_{fppf} \longrightarrow (\mathit{Sch}/Y)_{smooth}$,
$(\mathit{Sch}/X)_{fppf} \longrightarrow (\mathit{Sch}/Y)_{\acute{e}tale}$,
$(\mathit{Sch}/X)_{fppf} \longrightarrow (\mathit{Sch}/Y)_{Zar}$,
$(\mathit{Sch}/X)_{syntomic} \longrightarrow (\mathit{Sch}/Y)_{syntomic}$,
$(\mathit{Sch}/X)_{syntomic} \longrightarrow (\mathit{Sch}/Y)_{smooth}$,
$(\mathit{Sch}/X)_{syntomic} \longrightarrow (\mathit{Sch}/Y)_{\acute{e}tale}$,
$(\mathit{Sch}/X)_{syntomic} \longrightarrow (\mathit{Sch}/Y)_{Zar}$,
$(\mathit{Sch}/X)_{smooth} \longrightarrow (\mathit{Sch}/Y)_{smooth}$,
$(\mathit{Sch}/X)_{smooth} \longrightarrow (\mathit{Sch}/Y)_{\acute{e}tale}$,
$(\mathit{Sch}/X)_{smooth} \longrightarrow (\mathit{Sch}/Y)_{Zar}$,
$(\mathit{Sch}/X)_{\acute{e}tale}\longrightarrow (\mathit{Sch}/Y)_{\acute{e}tale}$,
$(\mathit{Sch}/X)_{\acute{e}tale}\longrightarrow (\mathit{Sch}/Y)_{Zar}$,
$(\mathit{Sch}/X)_{Zar} \longrightarrow (\mathit{Sch}/Y)_{Zar}$,
$(\mathit{Sch}/X)_{fppf} \longrightarrow Y_{\acute{e}tale}$,
$(\mathit{Sch}/X)_{syntomic} \longrightarrow Y_{\acute{e}tale}$,
$(\mathit{Sch}/X)_{smooth} \longrightarrow Y_{\acute{e}tale}$,
$(\mathit{Sch}/X)_{\acute{e}tale}\longrightarrow Y_{\acute{e}tale}$,
$(\mathit{Sch}/X)_{fppf} \longrightarrow Y_{Zar}$,
$(\mathit{Sch}/X)_{syntomic} \longrightarrow Y_{Zar}$,
$(\mathit{Sch}/X)_{smooth} \longrightarrow Y_{Zar}$,
$(\mathit{Sch}/X)_{\acute{e}tale}\longrightarrow Y_{Zar}$,
$(\mathit{Sch}/X)_{Zar} \longrightarrow Y_{Zar}$,
$X_{\acute{e}tale}\longrightarrow Y_{\acute{e}tale}$,
$X_{\acute{e}tale}\longrightarrow Y_{Zar}$,
$X_{Zar} \longrightarrow Y_{Zar}$,
In each case the underlying continuous functor $\mathit{Sch}/Y \to \mathit{Sch}/X$, or $Y_\tau \to \mathit{Sch}/X$ is the functor $Y'/Y \mapsto X \times _ Y Y'/X$. Namely, in the sections above we have seen the morphisms $f_{big} : (\mathit{Sch}/X)_\tau \to (\mathit{Sch}/Y)_\tau $ and $f_{small} : X_\tau \to Y_\tau $ for $\tau $ as above. We also have seen the morphisms of sites $\pi _ Y : (\mathit{Sch}/Y)_\tau \to Y_\tau $ for $\tau \in \{ {\acute{e}tale}, Zariski\} $. On the other hand, it is clear that the identity functor $(\mathit{Sch}/X)_\tau \to (\mathit{Sch}/X)_{\tau '}$ defines a morphism of sites when $\tau $ is a stronger topology than $\tau '$. Hence composing these gives the list of possible morphisms above.
Because of the simple description of the underlying functor it is clear that given morphisms of schemes $X \to Y \to Z$ the composition of two of the morphisms of sites above, e.g.,
\[ (\mathit{Sch}/X)_{\tau _0} \longrightarrow (\mathit{Sch}/Y)_{\tau _1} \longrightarrow (\mathit{Sch}/Z)_{\tau _2} \]
is the corresponding morphism of sites associated to the morphism of schemes $X \to Z$.
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