diff --git a/coherent.tex b/coherent.tex index cf5e7584a..6ce6faffb 100644 --- a/coherent.tex +++ b/coherent.tex @@ -888,7 +888,7 @@ \section{Cohomology and base change, II} \label{lemma-separated-case-relative-cech} Let $f : X \to S$ be a morphism of schemes. Let $\mathcal{F}$ be a quasi-coherent $\mathcal{O}_X$-module. -Assume $X$ and $S$ are quasi-compact and have affine diagonal +Assume $X$ is quasi-compact and $X$ and $S$ have affine diagonal (e.g., if $X$ and $S$ are separated). In this case we can compute $Rf_*\mathcal{F}$ as follows: \begin{enumerate} @@ -924,8 +924,8 @@ \section{Cohomology and base change, II} \begin{proof} Consider the resolution -$\mathcal{F} \to {\mathfrak C}^\bullet(\mathcal{U}, \mathcal{F})$ of -Cohomology, Lemma \ref{cohomology-lemma-covering-resolution}. +$\mathcal{F} \to {\mathfrak C}^\bullet(\mathcal{U}, \mathcal{F})$ +of Cohomology, Lemma \ref{cohomology-lemma-covering-resolution}. We have an equality of complexes $\check{\mathcal{C}}^\bullet(\mathcal{U}, f, \mathcal{F}) = f_*{\mathfrak C}^\bullet(\mathcal{U}, \mathcal{F})$ @@ -935,18 +935,13 @@ \section{Cohomology and base change, II} are affine by Morphisms, Lemma \ref{morphisms-lemma-affine-permanence}. Hence $R^qj_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p}$ as well as $R^qf_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p}$ -are zero for $q > 0$. By +are zero for $q > 0$ (Lemma \ref{lemma-relative-affine-vanishing}). +Using $f \circ j_{i_0 \ldots i_p} = f_{i_0 \ldots i_p}$ and +the spectral sequence of Cohomology, Lemma \ref{cohomology-lemma-relative-Leray} we conclude that -$Rj_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p} = -j_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p}$ and -$Rf_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p} = -f_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p}$. -Using that $Rf_* \circ Rj_{i_0 \ldots i_p *} = Rf_{i_0 \ldots i_p *}$ -(Cohomology, Lemma \ref{cohomology-lemma-higher-direct-images-compose}) -we find that -$Rf_*j_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p} = -f_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p}$. +$R^qf_*(j_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p}) = 0$ +for $q > 0$. Since the terms of the complex ${\mathfrak C}^\bullet(\mathcal{U}, \mathcal{F})$ are finite direct sums of the sheaves $j_{i_0 \ldots i_p *}\mathcal{F}_{i_0 \ldots i_p}$ @@ -954,9 +949,8 @@ \section{Cohomology and base change, II} (Derived Categories, Lemma \ref{derived-lemma-leray-acyclicity}) that $$ -Rf_* \mathcal{F} = -Rf_*{\mathfrak C}^\bullet(\mathcal{U}, \mathcal{F}) = -f_*{\mathfrak C}^\bullet(\mathcal{U}, \mathcal{F}) +Rf_* \mathcal{F} = f_*{\mathfrak C}^\bullet(\mathcal{U}, \mathcal{F}) = +\check{\mathcal{C}}^\bullet(\mathcal{U}, f, \mathcal{F}) $$ as desired. \end{proof}