diff --git a/duality.tex b/duality.tex index 97f8e71d4..a3434240e 100644 --- a/duality.tex +++ b/duality.tex @@ -5837,25 +5837,30 @@ \section{Relative dualizing complexes} \begin{definition} \label{definition-relative-dualizing-complex} Let $X \to S$ be a morphism of schemes which is flat and -locally of finite presentation. +locally of finite presentation. Let $W \subset X \times_S X$ +be any open such that the diagonal $\Delta_{X/S} : X \times_S X$ +factors through a closed immersion $\Delta : X \to W$. A {\it relative dualizing complex} is a pair $(K, \xi)$ consisting of an object $K \in D(\mathcal{O}_X)$ and a map $$ -\xi : \Delta_*\mathcal{O}_X \longrightarrow L\text{pr}_1^*K +\xi : \Delta_*\mathcal{O}_X \longrightarrow L\text{pr}_1^*K|_W $$ -in $D(\mathcal{O}_{X \times_S X})$ such that +in $D(\mathcal{O}_W)$ such that \begin{enumerate} \item $K$ is $S$-perfect (Derived Categories of Schemes, Definition \ref{perfect-definition-relatively-perfect}), and \item $\xi$ defines an isomorphism of $\Delta_*\mathcal{O}_X$ with -$R\SheafHom_{\mathcal{O}_{X \times_S X}}( -\Delta_*\mathcal{O}_X, L\text{pr}_1^*K)$. +$R\SheafHom_{\mathcal{O}_W}( +\Delta_*\mathcal{O}_X, L\text{pr}_1^*K|_W)$. \end{enumerate} \end{definition} \noindent +Observe that the category of pairs $(K, \xi)$ as in the +definition is independent of the choice of $W$. If $X \to S$ +is separated, then we can choose $W = X \times_S X$. We will reduce many of the arguments to the case of rings using the following lemma. @@ -5895,7 +5900,7 @@ \section{Relative dualizing complexes} \ref{perfect-lemma-quasi-coherence-internal-hom}. For the last one we observe that $L\text{pr}_1^*K$ is $S$-perfect (hence bounded below) and that $\Delta_*\mathcal{O}_X$ -is a pseudo-coherent object of $D(\mathcal{O}_{X \times_S X})$; +is a pseudo-coherent object of $D(\mathcal{O}_W)$; translated into algebra this means that $A$ is pseudo-coherent as an $A \otimes_R A$-module which follows from More on Algebra, Lemma @@ -5943,7 +5948,7 @@ \section{Relative dualizing complexes} \medskip\noindent Suppose we have an isomorphism $\alpha : K \to L$. Then $\alpha(\xi) = u \eta$ for some invertible section -$u \in H^0(X \times_S X, \Delta_*\mathcal{O}_X) = H^0(X, \mathcal{O}_X)$. +$u \in H^0(W, \Delta_*\mathcal{O}_X) = H^0(X, \mathcal{O}_X)$. (Because both $\eta$ and $\alpha(\xi)$ are generators of an invertible $\Delta_*\mathcal{O}_X$-module by assumption.) Hence after replacing $\alpha$ by $u^{-1}\alpha$ @@ -6018,41 +6023,40 @@ \section{Relative dualizing complexes} \medskip\noindent To finish the proof we have to construct the map $$ -\xi : \Delta_*\mathcal{O}_X \longrightarrow L\text{pr}_1^*K +\xi : \Delta_*\mathcal{O}_X \longrightarrow L\text{pr}_1^*K|_W $$ -in $D(\mathcal{O}_{X \times_S X})$ inducing an isomorphism from -$\Delta_*\mathcal{O}_X$ to $R\SheafHom_{\mathcal{O}_{X \times_S X}}( -\Delta_*\mathcal{O}_X, L\text{pr}_1^*K)$. We can use $\rho_U$ to -get isomorphisms +in $D(\mathcal{O}_W)$ inducing an isomorphism from $\Delta_*\mathcal{O}_X$ to +$R\SheafHom_{\mathcal{O}_W}(\Delta_*\mathcal{O}_X, L\text{pr}_1^*K|_W)$. +Since we may change $W$, we choose +$W = \bigcup_{U \in \mathcal{B}} U \times_S U$. +We can use $\rho_U$ to get isomorphisms $$ -R\SheafHom_{\mathcal{O}_{X \times_S X}}( -\Delta_*\mathcal{O}_X, L\text{pr}_1^*K)|_{U \times_S U} +R\SheafHom_{\mathcal{O}_W}( +\Delta_*\mathcal{O}_X, L\text{pr}_1^*K|_W)|_{U \times_S U} \xrightarrow{\rho_U} R\SheafHom_{\mathcal{O}_{U \times_S U}}( \Delta_*\mathcal{O}_U, L\text{pr}_1^*K_U) $$ -As $R\SheafHom_{\mathcal{O}_{X \times_S X}}( -\Delta_*\mathcal{O}_X, L\text{pr}_1^*K)$ -is supported on $\Delta(X)$ we conclude that its cohomology sheaves are zero -except in degree $0$. Moreover, we obtain isomorphisms +As $W$ is covered by the opens $U \times_S U$ +we conclude that the cohomology sheaves of +$R\SheafHom_{\mathcal{O}_W}(\Delta_*\mathcal{O}_X, L\text{pr}_1^*K|_W)$ +are zero except in degree $0$. Moreover, we obtain isomorphisms $$ +H^0\left(U \times_S U, R\SheafHom_{\mathcal{O}_W}(\Delta_*\mathcal{O}_X, +L\text{pr}_1^*K|_W)\right) +\xrightarrow{\rho_U} H^0\left((R\SheafHom_{\mathcal{O}_{U \times_S U}}( \Delta_*\mathcal{O}_U, L\text{pr}_1^*K_U)\right) -\xrightarrow{\rho_U} -H^0\left(R\SheafHom_{\mathcal{O}_{X \times_S X}}(\Delta_*\mathcal{O}_X, -L\text{pr}_1^*K)\right)|_{U \times_S U} $$ -Let $\tau_U$ in the RHS be the image of $\xi_U$ -viewed as an element of the LHS under this map. +Let $\tau_U$ in the LHS be an element mapping to $\xi_U$ under this map. The compatibilities between $\rho^U_{U'}$, $\xi_U$, $\xi_{U'}$, $\rho_U$, and $\rho_{U'}$ for $U' \subset U \subset X$ open $U', U \in \mathcal{B}$ -imply that $\tau_U|_{U' \times_S U'} = \tau_{U'}$. Since -$H^0(R\SheafHom_{\mathcal{O}_{X \times_S X}}(\Delta_*\mathcal{O}_X, -L\text{pr}_1^*K))$ is a sheaf we get a global section $\tau$. +imply that $\tau_U|_{U' \times_S U'} = \tau_{U'}$. +Thus we get a global section $\tau$ of the $0$th cohomology sheaf +$H^0(R\SheafHom_{\mathcal{O}_W}(\Delta_*\mathcal{O}_X, L\text{pr}_1^*K|_W))$. Since the other cohomology sheaves of -$R\SheafHom_{\mathcal{O}_{X \times_S X}}(\Delta_*\mathcal{O}_X, -L\text{pr}_1^*K)$ +$R\SheafHom_{\mathcal{O}_W}(\Delta_*\mathcal{O}_X, L\text{pr}_1^*K|_W)$ are zero, this global section $\tau$ determines a morphism $\xi$ as desired. Since the restriction of $\xi$ to $U \times_S U$ gives $\xi_U$, we see that it @@ -6079,17 +6083,22 @@ \section{Relative dualizing complexes} Consider the cartesian square $$ \xymatrix{ -X' \ar[d]_{\Delta'} \ar[r] & X \ar[d]^\Delta \\ +X' \ar[d]_{\Delta_{X'/S'}} \ar[r] & X \ar[d]^\Delta_{X/S} \\ X' \times_{S'} X' \ar[r]^{g' \times g'} & X \times_S X } $$ -We have +Choose $W \subset X \times_S X$ open such that $\Delta_{X/S}$ +factors through a closed immersion $\Delta : X \to W$. +Choose $W' \subset X' \times_{S'} X'$ open such that $\Delta_{X'/S'}$ +factors through a closed immersion $\Delta' : X \to W'$ +and such that $(g' \times g')(W') \subset W$. Let us still denote +$g' \times g' : W' \to W$ the induced morphism. We have $$ L(g' \times g')^*\Delta_*\mathcal{O}_X = \Delta'_*\mathcal{O}_{X'} \quad\text{and}\quad -L(g' \times g')^*L\text{pr}_1^*K = -L\text{pr}_1^*K' +L(g' \times g')^*L\text{pr}_1^*K|_W = +L\text{pr}_1^*K'|_{W'} $$ The first equality holds because $X$ and $X' \times_{S'} X'$ are tor independent over $X \times_S X$ (see for example @@ -6098,15 +6107,15 @@ \section{Relative dualizing complexes} (Cohomology, Lemma \ref{cohomology-lemma-derived-pullback-composition}). Thus $\xi' = L(g' \times g')^*\xi$ can be viewed as a map $$ -\xi' : \Delta'_*\mathcal{O}_{X'} \longrightarrow L\text{pr}_1^*K' +\xi' : \Delta'_*\mathcal{O}_{X'} \longrightarrow L\text{pr}_1^*K'|_{W'} $$ Having said this the proof of the lemma is straightforward. First, $K'$ is $S'$-perfect by Derived Categories of Schemes, Lemma \ref{perfect-lemma-base-change-relatively-perfect}. To check that $\xi'$ induces an isomorphism of $\Delta'_*\mathcal{O}_{X'}$ to -$R\SheafHom_{\mathcal{O}_{X' \times_{S'} X'}}( -\Delta'_*\mathcal{O}_{X'}, L\text{pr}_1^*K')$ +$R\SheafHom_{\mathcal{O}_{W'}}( +\Delta'_*\mathcal{O}_{X'}, L\text{pr}_1^*K'|_{W'})$ we may work affine locally. By Lemma \ref{lemma-relative-dualizing-complex-algebra} we reduce to the corresponding statement in algebra @@ -6167,6 +6176,8 @@ \section{Relative dualizing complexes} + + \input{chapters} \bibliography{my}