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
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
f, f', g, and g' correspond to cocontinuous functors u, u', v, and v' as in Sites, Lemma 7.21.1,
v \circ u' = u \circ v',
v and v' are continuous as well as cocontinuous,
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
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
u and u' are continuous and give rise to the morphisms f and f',
v and v' are cocontinuous giving rise to the morphisms g and g',
u \circ v = v' \circ u',
v and v' are continuous as well as cocontinuous, and
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
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