Lemma 103.8.2. Let $\mathcal{X}$ be an algebraic stack. Let $K$ be an object of $D(\mathcal{X}_{fppf})$ such that $R\Gamma (x, K) = 0$ for all objects $x$ of $\mathcal{X}$ lying over an affine scheme $U$ such that $U \to \mathcal{X}$ is flat. Then $H^ i(\mathcal{X}, K) = 0$ for all $i$.

**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 103.3. By Lemma 103.4.2 part (2)(b) our assumption means that $g^{-1}K$ has vanishing cohomology over every object of $\mathcal{X}_{flat, fppf}$ which lies over an affine scheme. Since every object $x$ of $\mathcal{X}_{flat, fppf}$ has a covering by such objects, we conclude that $g^{-1}K$ has vanishing cohomology sheaves, i.e., we conclude $g^{-1}K = 0$. Then of course $R\Gamma (\mathcal{X}_{flat, fppf}, g^{-1}K) = 0$ which in turn implies what we want by Lemma 103.4.2 part (2)(a).
$\square$

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