Lemma 60.25.5 (07QA)—The Stacks project

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Table of contents
Part 3: Topics in Scheme Theory
Chapter 60: Crystalline Cohomology
Section 60.25: Pulling back along purely inseparable maps
Lemma 60.25.5 (cite)

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Lemma 60.25.5. With notations and assumptions as in Lemma 60.25.4 the map

$f^* : H^ i(\text{Cris}(X/S), \mathcal{F}) \longrightarrow H^ i(\text{Cris}(X'/S), \mathcal{F}')$

has kernel and cokernel annihilated by $q^{i + 1}$ .

Proof. This follows from the fact that $E$ has nonzero cohomology sheaves in degrees $-1$ and up, so that the spectral sequence $H^ a(\mathcal{H}^ b(E)) \Rightarrow H^{a + b}(E)$ converges. This combined with the long exact cohomology sequence associated to a distinguished triangle gives the bound. $\square$

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