diff --git a/dualizing.tex b/dualizing.tex index 73a436f4c..f4b651972 100644 --- a/dualizing.tex +++ b/dualizing.tex @@ -2058,20 +2058,17 @@ \section{Trivial duality for a ring map} M \longmapsto \Hom_A(B, M) $$ This functor is left exact and has a derived extension -$R\Hom(B, -) : D(A) \to D(B)$. If $f_* : D(B) \to D(A)$ is the restriction -functor, then $f_*R\Hom(B, K) = R\Hom_A(B, K)$ for every $K \in D(A)$. -Since $R\Hom_A(A, K) = K$, the map $A \to B$ induces a canonical map -$f_*R\Hom(B, K) \to K$ in $D(A)$ functorial in $K$. +$R\Hom(B, -) : D(A) \to D(B)$. \begin{lemma} \label{lemma-right-adjoint} Let $A \to B$ be a ring homomorphism. The functor $R\Hom(B, -)$ -constructed above is the right adjoint to the restriction functor -$f_* : D(B) \to D(A)$. +constructed above is right adjoint to the restriction functor +$D(B) \to D(A)$. \end{lemma} \begin{proof} -This is a consequence of the fact that $f_*$ and $\Hom_A(B, -)$ are +This is a consequence of the fact that restriction and $\Hom_A(B, -)$ are adjoint functors by Algebra, Lemma \ref{algebra-lemma-adjoint-tensor-restrict}. See Derived Categories, Lemma \ref{derived-lemma-derived-adjoint-functors}. \end{proof} @@ -2089,13 +2086,19 @@ \section{Trivial duality for a ring map} \begin{lemma} \label{lemma-RHom-ext} -Let $A \to B$ be a ring homomorphism. For $K$ in $D(A)$ we have -$f_*R\Hom(B, K) = R\Hom_A(B, K)$ where $f_* : D(B) \to D(A)$ is -restriction. In particular $R^q\Hom(B, K) = \Ext_A^q(B, K)$. +Let $\varphi : A \to B$ be a ring homomorphism. For $K$ in $D(A)$ we have +$$ +\varphi_*R\Hom(B, K) = R\Hom_A(B, K) +$$ +where $\varphi_* : D(B) \to D(A)$ is restriction. In particular +$R^q\Hom(B, K) = \Ext_A^q(B, K)$. \end{lemma} \begin{proof} -Omitted, but see above. +Choose a K-injective complex $I^\bullet$ representing $K$. +Then $R\Hom(B, K)$ is represented by the complex $\Hom_A(B, I^\bullet)$ +of $B$-modules. Since this complex, as a complex of $A$-modules, +represents $R\Hom_A(B, K)$ we see that the lemma is true. \end{proof} \noindent @@ -2285,6 +2288,118 @@ \section{Trivial duality for a ring map} +\section{Base change for trivial duality} +\label{section-base-change-trivial-duality} + +\noindent +In this section we consider a cocartesian square of rings +$$ +\xymatrix{ +A \ar[r]_\alpha & A' \\ +R \ar[u]^\varphi \ar[r]^\rho & R' \ar[u]_{\varphi'} +} +$$ +In other words, we have $A' = A \otimes_R R'$. If $A$ and $R'$ +are {\bf tor independent over} $R$ then there is a canonical base change map +\begin{equation} +\label{equation-base-change} +R\Hom(A, K) \otimes_A^\mathbf{L} A' +\longrightarrow +R\Hom(A', K \otimes_R^\mathbf{L} R') +\end{equation} +in $D(A')$ functorial for $K$ in $D(R)$. Namely, by the adjointness +of Lemma \ref{lemma-right-adjoint} such an arrow is the same thing as a map +$$ +\varphi'_*\left(R\Hom(A, K) \otimes_A^\mathbf{L} A'\right) +\longrightarrow +K \otimes_R^\mathbf{L} R' +$$ +in $D(R')$ where $\varphi'_* : D(A') \to D(R')$ is the restriction functor. +We may apply +More on Algebra, Lemma \ref{more-algebra-lemma-base-change-comparison} +to the left hand side to get that this is the same thing as a map +$$ +\varphi_*(R\Hom(A, K)) \otimes_R^\mathbf{L} R' +\longrightarrow +K \otimes_R^\mathbf{L} R' +$$ +in $D(R')$ where $\varphi_* : D(A) \to D(R)$ is the restriction functor. +For this we can choose $can \otimes^\mathbf{L} \text{id}_{R'}$ +where $can : \varphi_*(R\Hom_R(A, K)) \to K$ is the +counit of the adjunction between $R\Hom(A, -)$ and $\varphi_*$. + +\begin{lemma} +\label{lemma-check-base-change-is-iso} +In the situation above, the map (\ref{equation-base-change}) +is an isomorphism if and only if the map +$$ +R\Hom_R(A, K) \otimes_R^\mathbf{L} R' +\longrightarrow +R\Hom_R(A, K \otimes_R^\mathbf{L} R') +$$ +of More on Algebra, Lemma +\ref{more-algebra-lemma-internal-hom-diagonal-better} is an isomorphism. +\end{lemma} + +\begin{proof} +To see that the map is an isomorphism, it suffices to prove it +is an isomorphism after applying $\varphi'_*$. +Applying the functor $\varphi'_*$ to (\ref{equation-base-change}) +and using that $A' = A \otimes_R^\mathbf{L} R'$ +we obtain the base change map +$R\Hom_R(A, K) \otimes_R^\mathbf{L} R' \to +R\Hom_{R'}(A \otimes_R^\mathbf{L} R', K \otimes_R^\mathbf{L} R')$ +for derived hom of +More on Algebra, Equation (\ref{more-algebra-equation-base-change-RHom}). +Unwinding the left and right hand side exactly as in the proof of +More on Algebra, Lemma \ref{more-algebra-lemma-base-change-RHom} +and in particular using +More on Algebra, Lemma \ref{more-algebra-lemma-upgrade-adjoint-tensor-RHom} +gives the desired result. +\end{proof} + +\begin{lemma} +\label{lemma-flat-bc-surjection} +Let $R \to A$ and $R \to R'$ be ring maps and $A' = A \otimes_R R'$. +Assume +\begin{enumerate} +\item $A$ is pseudo-coherent as an $R$-module, +\item $R'$ has finite tor dimension as an $R$-module (for example +$R \to R'$ is flat), +\item $A$ and $R'$ are tor independent over $R$. +\end{enumerate} +Then (\ref{equation-base-change}) is an isomorphism for $K \in D^+(R)$. +\end{lemma} + +\begin{proof} +Follows from Lemma \ref{lemma-check-base-change-is-iso} and +More on Algebra, Lemma +\ref{more-algebra-lemma-internal-hom-evaluate-tensor-isomorphism} part (4). +\end{proof} + +\begin{lemma} +\label{lemma-bc-surjection} +Let $R \to A$ and $R \to R'$ be ring maps and $A' = A \otimes_R R'$. +Assume +\begin{enumerate} +\item $A$ is perfect as an $R$-module, +\item $A$ and $R'$ are tor independent over $R$. +\end{enumerate} +Then (\ref{equation-base-change}) is an isomorphism for all $K \in D(R)$. +\end{lemma} + +\begin{proof} +Follows from Lemma \ref{lemma-check-base-change-is-iso} and +More on Algebra, Lemma +\ref{more-algebra-lemma-internal-hom-evaluate-tensor-isomorphism} part (1). +\end{proof} + + + + + + + \section{Dualizing complexes} \label{section-dualizing} @@ -4103,58 +4218,6 @@ \section{Upper shriek algebraically} \ref{lemma-dualizing-quotient}, \end{proof} -\begin{lemma} -\label{lemma-flat-bc-surjection} -Let $\varphi : R \to A$ be a surjective homomorphism of Noetherian rings. -Let $R \to R'$ be a flat homomorphism of Noetherian rings. -Let $\varphi' : R' \to A' = A \otimes_R R'$ be the map induced by $\varphi$. -Then we have a functorial maps -$$ -\varphi^!(K) \otimes_A^\mathbf{L} A' -\longrightarrow -(\varphi')^!(K \otimes_R^\mathbf{L} R') -$$ -for $K$ in $D(R)$ which are isomorphisms for $K \in D^+(R)$. -\end{lemma} - -\begin{proof} -Let $K \in D(R)$. Choose a K-injective complex $I^\bullet$ representing -$K$. Then $I^\bullet \otimes_R R'$ represents $K \otimes_R^\mathbf{L} R'$. -Choose a quasi-isomorphism $I^\bullet \otimes_R R' \to J^\bullet$ -where $J^\bullet$ is a K-injective complex of $R'$-modules. -Then there is a canonical map -$$ -\Hom_R(A, I^\bullet) \otimes_R R' \to -\Hom_{R'}(A \otimes_R R', I^\bullet \otimes_R R') \to -\Hom_{R'}(A', J^\bullet) -$$ -Since $R \to R'$ is flat, the map $A \to A'$ is flat and -$D(A) \to D(A')$ is given by $L^\bullet \mapsto L^\bullet \otimes_R R'$ -on the level of complexes. Hence the source of the arrow, -viewed as a complex of $A'$-modules represents -$\varphi^!(K) \otimes_A^\mathbf{L} A'$. -Similarly, the target viewed as a complex of $A'$-modules -represents $(\varphi')^!(K \otimes_R^\mathbf{L} R')$. -This defines the natural transformation of the lemma. - -\medskip\noindent -To see that the map is an isomorphism for some $K$ we may do so after -applying the restriction functor $D(A') \to D(R)$. -Since $J^\bullet$ is K-injective as a complex of $R$-modules -(More on Algebra, Lemma \ref{more-algebra-lemma-K-injective-flat}) -and since $\Hom_{R'}(A', -) = \Hom_R(A, -)$ we see that -$\Hom_{R'}(A', J^\bullet)$ represents -$R\Hom_R(A, K \otimes_R^\mathbf{L} R')$ in $D(R)$. -The right hand side represents $R\Hom(A, K) \otimes_R^\mathbf{L} R'$ -in $D(R)$. Checking the definitions we see our map is the map -$$ -R\Hom(A, K) \otimes_R^\mathbf{L} R' \to R\Hom(A, K \otimes_R^\mathbf{L} R') -$$ -of More on Algebra, Lemma -\ref{more-algebra-lemma-internal-hom-evaluate-tensor-isomorphism}. -In this way we conclude the final assertion of the lemma is true. -\end{proof} - \begin{lemma} \label{lemma-flat-bc} Let $R \to R'$ be a flat homomorphism of Noetherian rings. @@ -4169,133 +4232,22 @@ \section{Upper shriek algebraically} \end{lemma} \begin{proof} -We can choose a factorization $R \to P \to A$ where $P$ is a polynomial -ring over $R$. This gives a corresponding factorization -$R' \to P' \to A'$ by base change. Since we have -$(K \otimes_R^\mathbf{L} P) \otimes_P^\mathbf{L} P' = +Choose a factorization $R \to P \to A$ where $P$ is a polynomial ring over $R$. +This gives a corresponding factorization $R' \to P' \to A'$ by base change. +Since we have $(K \otimes_R^\mathbf{L} P) \otimes_P^\mathbf{L} P' = (K \otimes_R^\mathbf{L} R') \otimes_{R'}^\mathbf{L} P'$ by More on Algebra, Lemma \ref{more-algebra-lemma-double-base-change} -it suffices to prove the statement for -$P \to A$ and $P \to P'$. This case is proved in -Lemma \ref{lemma-flat-bc-surjection}. -\end{proof} - -\begin{lemma} -\label{lemma-bc-surjection} -Let $\varphi : R \to A$ be a surjective homomorphism of Noetherian rings. -Let $R \to R'$ be a homomorphism of Noetherian rings. Assume -\begin{enumerate} -\item $A$ is a perfect $R$-module, -\item $R'$ and $A$ are Tor independent over $R$. -\end{enumerate} -Let $\varphi' : R' \to A' = A \otimes_R R'$ be the map induced by $\varphi$. -Then we have a functorial isomorphism +it suffices to construct maps $$ -\varphi^!(K) \otimes_A^\mathbf{L} A' = -(\varphi')^!(K \otimes_R^\mathbf{L} R') -$$ -for $K$ in $D(R)$. -\end{lemma} - -\begin{proof} -Because $A$ is a perfect $R$-module, we see that -$A \otimes_R^\mathbf{L} R'$ is a perfect object of $D(R')$, see -More on Algebra, Lemma \ref{more-algebra-lemma-pull-perfect}. -By assumption (2) we have $A' = A \otimes_R^\mathbf{L} R'$ -is a perfect $R'$-module. Thus it follows from -Lemma \ref{lemma-RHom-is-tensor-special} -that $\varphi^!$ and $(\varphi')^!$ are given by -derived tensor product with $N = R\Hom(A, R)$ and -$N' = R\Hom(A', R')$. Hence it suffices to show that -$$ -(K \otimes_R^\mathbf{L} N) \otimes_A^\mathbf{L} A' = -(K \otimes_R^\mathbf{L} R') \otimes_{R'}^\mathbf{L} N' -$$ -functorially for $K \in D(R)$. By transitivity of tensor -functors (More on Algebra, Lemma \ref{more-algebra-lemma-double-base-change}) -we conclude that it suffices to construct -an isomorphism $N \otimes_A^\mathbf{L} A' \to N'$ -in $D(A')$. - -\medskip\noindent -Choose a Tate resolution $R \to (E, \text{d}) \to A$ with $E^0 = R$ as in -Divided Power Algebra, Lemma \ref{dpa-lemma-tate-resolution}. -Then $E^n = 0$ for $n > 0$ and $E^n$ is a finite free $R$-module -for all $n$. Write $E' = E \otimes_R R'$ with induced differential $\text{d}$. -Since $E^\bullet \to A$ is a flat resolution of $A$ as an $R$-module, -we see that the map $E' \to A'$ is a quasi-isomorphism by assumption (2). -The diagram -$$ -\xymatrix{ -D(A) \ar[r]_{- \otimes_A^\mathbf{L} A'} & D(A') \\ -D(E, \text{d}) -\ar[u]^{- \otimes_E^\mathbf{L} A} \ar[r]^{- \otimes_E^\mathbf{L} E'} & -D(E', \text{d}) \ar[u]_{- \otimes_{E'}^\mathbf{L} A'} -} -$$ -is commutative (each of the functors as well as the compositions -are adjoints to restriction the restriction functors satisfy -the corresponding commutativity). The vertical arrows -are equivalences because $E \to A$ and $E' \to A'$ -are quasi-isomorphisms -(Differential Graded Algebra, Lemma \ref{dga-lemma-qis-equivalence}). - -\medskip\noindent -Putting this together with Lemma \ref{lemma-RHom-dga} we find that it suffices -to construct an isomorphism -\begin{equation} -\label{equation-bc-map-algebraic} -R\Hom(E, R) \otimes_E^\mathbf{L} E' +R\Hom(A, K \otimes_R^\mathbf{L} P[n]) \otimes_A^\mathbf{L} A' \longrightarrow -R\Hom(E', R') -\end{equation} -in $D(E')$. - -\medskip\noindent -By Differential Graded Algebra, Lemma -\ref{dga-lemma-compute-derived-restriction} -the differential graded $E$-module -$$ -H = \Hom_{\text{Mod}^{dg}_R}(E, R) -$$ -represents $R\Hom(E, R)$ in $D(E, \text{d})$. -The underlying complex of $R$-modules is -$H^\bullet = \Hom^\bullet_R(E^\bullet, K^\bullet)$. This is a -K-flat complex of $R$-modules by -More on Algebra, Lemma \ref{more-algebra-lemma-hom-complex-K-flat} -and our assumption that $E^\bullet$ is perfect. -By Differential Graded Algebra, Lemma \ref{dga-lemma-base-change-K-flat} -we see that the LHS of (\ref{equation-bc-map-algebraic}) -is represented by $H \otimes_R R'$ whose -underlying complex of $R'$-modules is $H^\bullet \otimes_R R'$. - -\medskip\noindent -Arguing as above we find that -$$ -H' = \Hom_{\text{Mod}^{dg}_R}(E', R') +R\Hom(A', (K \otimes_R^\mathbf{L} P[n]) \otimes_P^\mathbf{L} P') $$ -represents the RHS of (\ref{equation-bc-map-algebraic}) -and that as a complex of $R'$-modules we have -$$ -(H')^\bullet = -\Hom^\bullet_{R'}((E')^\bullet, R') = -\Hom^\bullet_{R'}(E^\bullet \otimes_R R', R') = -\Hom^\bullet_R(E^\bullet, R') -$$ -The last equality is proved by applying -Algebra, Lemma \ref{algebra-lemma-adjoint-tensor-restrict} -to the terms making up the Hom complex. - -\medskip\noindent -We define our map (\ref{equation-bc-map-algebraic}) to be the map -$$ -H \otimes_R R' \longrightarrow H' -$$ -defined by the following rule. An element $f$ of $H^n$ is an $R$-linear -map from $E^{-n}$ to $R$. Thus we send $f \otimes \lambda$ to -$\lambda f'$ where $f' : E^{-n} \to R'$ is the composition of -$f$ with $R \to R'$. It is immediately clear that this is -an isomorphism of $E'$-modules which finishes the proof. +functorial in $K$. For this we use the map (\ref{equation-base-change}) +constructed in Section \ref{section-base-change-trivial-duality} +for $P, A, P', A'$. +The map is an isomorphism for $K \in D^+(R)$ by +Lemma \ref{lemma-flat-bc-surjection}. \end{proof} \begin{lemma} @@ -4315,28 +4267,33 @@ \section{Upper shriek algebraically} \end{lemma} \begin{proof} -We can choose a factorization $R \to P \to A$ where $P$ is a polynomial -ring over $R$ and then $A$ is a perfect $P$-module, see +We may choose a factorization $R \to P \to A$ where $P$ +is a polynomial ring over $R$ such that $A$ is a perfect $P$-module, see More on Algebra, Lemma \ref{more-algebra-lemma-perfect-ring-map}. -This gives a corresponding factorization -$R' \to P' \to A'$ by base change. Since we have -$(K \otimes_R^\mathbf{L} P) \otimes_P^\mathbf{L} P' = +This gives a corresponding factorization $R' \to P' \to A'$ by base change. +Since we have $(K \otimes_R^\mathbf{L} P) \otimes_P^\mathbf{L} P' = (K \otimes_R^\mathbf{L} R') \otimes_{R'}^\mathbf{L} P'$ by More on Algebra, Lemma \ref{more-algebra-lemma-double-base-change} -it suffices to prove the statement for -$P \to A$ and $P \to P'$. -By Lemma \ref{lemma-bc-surjection} -it suffices to prove that $A$ is a perfect $P$-module -and that $A$ and $P'$ are tor independent over $P$. -The $P$-module $A$ is a perfect as seen above. -For tor independence +it suffices to construct maps +$$ +R\Hom(A, K \otimes_R^\mathbf{L} P[n]) \otimes_A^\mathbf{L} A' +\longrightarrow +R\Hom(A', (K \otimes_R^\mathbf{L} P[n]) \otimes_P^\mathbf{L} P') +$$ +functorial in $K$. We have $$ A \otimes_P^\mathbf{L} P' = A \otimes_R^\mathbf{L} R' = A' $$ The first equality by More on Algebra, Lemma \ref{more-algebra-lemma-base-change-comparison} applied to $R, R', P, P'$. The second equality because -$A$ and $R'$ are tor independent over $R$. +$A$ and $R'$ are tor independent over $R$. Hence $A$ and $P'$ are +tor independent over $P$ and we can use the map (\ref{equation-base-change}) +constructed in Section \ref{section-base-change-trivial-duality} for +$P, A, P', A'$ +get the desired arrow. By Lemma \ref{lemma-bc-surjection} +to finish the proof it suffices to prove that $A$ is a perfect $P$-module +which we saw above. \end{proof} \begin{lemma}