Lemma 104.4.2. Let \mathcal{X} be an algebraic stack. Notation as in Lemma 104.3.1.
For K in D(\mathcal{X}_{\acute{e}tale}) we have
R\Gamma (\mathcal{X}_{\acute{e}tale}, K) = R\Gamma (\mathcal{X}_{lisse,{\acute{e}tale}}, g^{-1}K), and
R\Gamma (x, K) = R\Gamma (\mathcal{X}_{lisse,{\acute{e}tale}}/x, g^{-1}K) for any object x of \mathcal{X}_{lisse,{\acute{e}tale}}.
For K in D(\mathcal{X}_{fppf}) we have
R\Gamma (\mathcal{X}_{fppf}, K) = R\Gamma (\mathcal{X}_{flat,fppf}, g^{-1}K), and
H^ p(x, K) = R\Gamma (\mathcal{X}_{flat,fppf}/x, g^{-1}K) for any object x of \mathcal{X}_{flat,fppf}.
In both cases, the same holds for modules, since we have g^{-1} = g^* and there is no difference in computing cohomology by Cohomology on Sites, Lemma 21.20.7.
Proof.
We prove this for the comparison between the flat-fppf site with the fppf site; the case of the lisse-étale site is exactly the same. By Lemma 104.4.1 we have Lg_!\mathbf{Z} = \mathbf{Z}. Then we obtain
\begin{align*} R\Gamma (\mathcal{X}_{fppf}, K) & = R\mathop{\mathrm{Hom}}\nolimits (\mathbf{Z}, K) \\ & = R\mathop{\mathrm{Hom}}\nolimits (Lg_!\mathbf{Z}, K) \\ & = R\mathop{\mathrm{Hom}}\nolimits (\mathbf{Z}, g^{-1}K) \\ & = R\Gamma (\mathcal{X}_{lisse,{\acute{e}tale}}, g^{-1}K) \end{align*}
This proves (1)(a). Part (1)(b) follows from part (1)(a). Namely, if x lies over the scheme U, then the site \mathcal{X}_{\acute{e}tale}/x is equivalent to (\mathit{Sch}/U)_{\acute{e}tale} and \mathcal{X}_{lisse,{\acute{e}tale}} is equivalent to U_{lisse, {\acute{e}tale}}.
\square
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