Processing math: 100%

The Stacks project

18.41 Lower shriek for modules

In this section we extend the construction of g_! discussed in Section 18.16 to the case of sheaves of modules.

Lemma 18.41.1. Let u : \mathcal{C} \to \mathcal{D} be a continuous and cocontinuous functor between sites. Denote g : \mathop{\mathit{Sh}}\nolimits (\mathcal{C}) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{D}) the associated morphism of topoi. Let \mathcal{O}_\mathcal {D} be a sheaf of rings on \mathcal{D}. Set \mathcal{O}_\mathcal {C} = g^{-1}\mathcal{O}_\mathcal {D}. Hence g becomes a morphism of ringed topoi with g^* = g^{-1}. In this case there exists a functor

g_! : \textit{Mod}(\mathcal{O}_\mathcal {C}) \longrightarrow \textit{Mod}(\mathcal{O}_\mathcal {D})

which is left adjoint to g^*.

Proof. Let U be an object of \mathcal{C}. For any \mathcal{O}_\mathcal {D}-module \mathcal{G} we have

\begin{align*} \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_\mathcal {C}}(j_{U!}\mathcal{O}_ U, g^{-1}\mathcal{G}) & = g^{-1}\mathcal{G}(U) \\ & = \mathcal{G}(u(U)) \\ & = \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_\mathcal {D}}(j_{u(U)!}\mathcal{O}_{u(U)}, \mathcal{G}) \end{align*}

because g^{-1} is described by restriction, see Sites, Lemma 7.21.5. Of course a similar formula holds a direct sum of modules of the form j_{U!}\mathcal{O}_ U. By Homology, Lemma 12.29.6 and Lemma 18.28.8 we see that g_! exists. \square

Remark 18.41.2. Warning! Let u : \mathcal{C} \to \mathcal{D}, g, \mathcal{O}_\mathcal {D}, and \mathcal{O}_\mathcal {C} be as in Lemma 18.41.1. In general it is not the case that the diagram

\xymatrix{ \textit{Mod}(\mathcal{O}_\mathcal {C}) \ar[r]_{g_!} \ar[d]_{forget} & \textit{Mod}(\mathcal{O}_\mathcal {D}) \ar[d]^{forget} \\ \textit{Ab}(\mathcal{C}) \ar[r]^{g^{Ab}_!} & \textit{Ab}(\mathcal{D}) }

commutes (here g^{Ab}_! is the one from Lemma 18.16.2). There is a transformation of functors

g_!^{Ab} \circ forget \longrightarrow forget \circ g_!

From the proof of Lemma 18.41.1 we see that this is an isomorphism if and only if g^{Ab}_!j_{U!}\mathcal{O}_ U \to g_!j_{U!}\mathcal{O}_ U is an isomorphism for all objects U of \mathcal{C}. Since we have g_!j_{U!}\mathcal{O}_ U = j_{u(U)!}\mathcal{O}_{u(U)} this holds if and only if

g^{Ab}_!j_{U!}\mathcal{O}_ U \longrightarrow j_{u(U)!}\mathcal{O}_{u(U)}

is an isomorphism for all objects U of \mathcal{C}. Note that for such a U we obtain a commutative diagram

\xymatrix{ \mathcal{C}/U \ar[r]_-{u'} \ar[d]_{j_ U} & \mathcal{D}/u(U) \ar[d]^{j_{u(U)}} \\ \mathcal{C} \ar[r]^ u & \mathcal{D} }

of cocontinuous functors of sites, see Sites, Lemma 7.28.4 and therefore g^{Ab}_!j_{U!} = j_{u(U)!}(g')^{Ab}_! where g' : \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{D}/u(U)) is the morphism of topoi induced by the cocontinuous functor u'. Hence we see that g_! = g^{Ab}_! if the canonical map

18.41.2.1
\begin{equation} \label{sites-modules-equation-compare-on-localizations} (g')^{Ab}_!\mathcal{O}_ U \longrightarrow \mathcal{O}_{u(U)} \end{equation}

is an isomorphism for all objects U of \mathcal{C}.

The following two results are of a slightly different nature.

Lemma 18.41.3. Assume given a commutative diagram

\xymatrix{ (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}'), \mathcal{O}_{\mathcal{C}'}) \ar[r]_{(g', (g')^\sharp )} \ar[d]_{(f', (f')^\sharp )} & (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}), \mathcal{O}_\mathcal {C}) \ar[d]^{(f, f^\sharp )} \\ (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}'), \mathcal{O}_{\mathcal{D}'}) \ar[r]^{(g, g^\sharp )} & (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}), \mathcal{O}_\mathcal {D}) }

of ringed topoi. Assume

  1. f, f', g, and g' correspond to cocontinuous functors u, u', v, and v' as in Sites, Lemma 7.21.1,

  2. v \circ u' = u \circ v',

  3. v and v' are continuous as well as cocontinuous,

  4. for any object V' of \mathcal{D}' the functor {}^{u'}_{V'}\mathcal{I} \to {}^{\ \ \ u}_{v(V')}\mathcal{I} given by v is cofinal, and

  5. g^{-1}\mathcal{O}_{\mathcal{D}} = \mathcal{O}_{\mathcal{D}'} and (g')^{-1}\mathcal{O}_{\mathcal{C}} = \mathcal{O}_{\mathcal{C}'}.

Then we have f'_* \circ (g')^* = g^* \circ f_* and g'_! \circ (f')^{-1} = f^{-1} \circ g_! on modules.

Proof. We have (g')^*\mathcal{F} = (g')^{-1}\mathcal{F} and g^*\mathcal{G} = g^{-1}\mathcal{G} because of condition (5). Thus the first equality follows immediately from the corresponding equality in Sites, Lemma 7.28.6. Since the left adjoint functors g_! and g'_! to g^* and (g')^* exist by Lemma 18.41.1 we see that the second equality follows by uniqueness of adjoint functors. \square

Lemma 18.41.4. Consider a commutative diagram

\xymatrix{ (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}'), \mathcal{O}_{\mathcal{C}'}) \ar[r]_{(g', (g')^\sharp )} \ar[d]_{(f', (f')^\sharp )} & (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}), \mathcal{O}_\mathcal {C}) \ar[d]^{(f, f^\sharp )} \\ (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}'), \mathcal{O}_{\mathcal{D}'}) \ar[r]^{(g, g^\sharp )} & (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}), \mathcal{O}_\mathcal {D}) }

of ringed topoi and suppose we have functors

\xymatrix{ \mathcal{C}' \ar[r]_{v'} & \mathcal{C} \\ \mathcal{D}' \ar[r]^ v \ar[u]^{u'} & \mathcal{D} \ar[u]_ u }

such that (with notation as in Sites, Sections 7.14 and 7.21) we have

  1. u and u' are continuous and give rise to the morphisms f and f',

  2. v and v' are cocontinuous giving rise to the morphisms g and g',

  3. u \circ v = v' \circ u',

  4. v and v' are continuous as well as cocontinuous, and

  5. g^{-1}\mathcal{O}_{\mathcal{D}} = \mathcal{O}_{\mathcal{D}'} and (g')^{-1}\mathcal{O}_{\mathcal{C}} = \mathcal{O}_{\mathcal{C}'}.

Then f'_* \circ (g')^* = g^* \circ f_* and g'_! \circ (f')^{-1} = f^{-1} \circ g_! on modules.

Proof. We have (g')^*\mathcal{F} = (g')^{-1}\mathcal{F} and g^*\mathcal{G} = g^{-1}\mathcal{G} because of condition (5). Thus the first equality follows immediately from the corresponding equality in Sites, Lemma 7.28.7. Since the left adjoint functors g_! and g'_! to g^* and (g')^* exist by Lemma 18.41.1 we see that the second equality follows by uniqueness of adjoint functors. \square


Comments (0)


Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.