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The Stacks project

96.8 Representable categories

In this short section we compare our definitions with what happens in case the algebraic stacks in question are representable.

Lemma 96.8.1. Let S be a scheme. Let \mathcal{X} be a category fibred in groupoids over (\mathit{Sch}/S). Assume \mathcal{X} is representable by a scheme X. For \tau \in \{ Zar,\linebreak[0] {\acute{e}tale},\linebreak[0] smooth,\linebreak[0] syntomic,\linebreak[0] fppf\} there is a canonical equivalence

(\mathcal{X}_\tau , \mathcal{O}_\mathcal {X}) = ((\mathit{Sch}/X)_\tau , \mathcal{O}_ X)

of ringed sites.

Proof. This follows by choosing an equivalence (\mathit{Sch}/X)_\tau \to \mathcal{X} of categories fibred in groupoids over (\mathit{Sch}/S)_{fppf} and using the functoriality of the construction \mathcal{X} \leadsto \mathcal{X}_\tau . \square

Lemma 96.8.2. Let S be a scheme. Let f : \mathcal{X} \to \mathcal{Y} be a morphism of categories fibred in groupoids over S. Assume \mathcal{X}, \mathcal{Y} are representable by schemes X, Y. Let f : X \to Y be the morphism of schemes corresponding to f. For \tau \in \{ Zar,\linebreak[0] {\acute{e}tale},\linebreak[0] smooth,\linebreak[0] syntomic,\linebreak[0] fppf\} the morphism of ringed topoi f : (\mathop{\mathit{Sh}}\nolimits (\mathcal{X}_\tau ), \mathcal{O}_\mathcal {X}) \to (\mathop{\mathit{Sh}}\nolimits (\mathcal{Y}_\tau ), \mathcal{O}_\mathcal {Y}) agrees with the morphism of ringed topoi f : (\mathop{\mathit{Sh}}\nolimits ((\mathit{Sch}/X)_\tau ), \mathcal{O}_ X) \to (\mathop{\mathit{Sh}}\nolimits ((\mathit{Sch}/Y)_\tau ), \mathcal{O}_ Y) via the identifications of Lemma 96.8.1.

Proof. Follows by unwinding the definitions. \square


Comments (2)

Comment #2259 by Neeraj Deshmukh on

There seems to be a minor typo in lemma 80.8.2. The morphism f is given on the same ringed topos.


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