Lemma 104.8.1. Let \mathcal{X} be an algebraic stack. Let K be an object of D(\mathcal{X}_{fppf}) whose cohomology sheaves are parasitic. Then R\Gamma (x, K) = 0 for all objects x of \mathcal{X} lying over a scheme U such that U \to \mathcal{X} is flat.
Proof. Denote g : \mathop{\mathit{Sh}}\nolimits (\mathcal{X}_{flat, fppf}) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{X}_{fppf}) the morphism of topoi discussed in Section 104.3. Let x be an object of \mathcal{X} lying over a scheme U such that U \to \mathcal{X} is flat, i.e., x is an object of \mathcal{X}_{flat, fppf}. By Lemma 104.4.2 part (2)(b) we have R\Gamma (x, K) = R\Gamma (\mathcal{X}_{flat, fppf}/x, g^{-1}K). However, our assumption means that the cohomology sheaves of the object g^{-1}K of D(\mathcal{X}_{flat, fppf}) are zero, see Cohomology of Stacks, Definition 103.9.1. Hence g^{-1}K = 0 and we win. \square
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