91.10 Modules on first order thickenings of ringed topoi
In this section we discuss some preliminaries to the deformation theory of modules. Let $i : (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}, \mathcal{O}) \to (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}), \mathcal{O}')$ be a first order thickening of ringed topoi. We will freely use the notation introduced in Section 91.9, in particular we will identify the underlying topological topoi. In this section we consider short exact sequences
91.10.0.1
\begin{equation} \label{defos-equation-extension-ringed-topoi} 0 \to \mathcal{K} \to \mathcal{F}' \to \mathcal{F} \to 0 \end{equation}
of $\mathcal{O}'$-modules, where $\mathcal{F}$, $\mathcal{K}$ are $\mathcal{O}$-modules and $\mathcal{F}'$ is an $\mathcal{O}'$-module. In this situation we have a canonical $\mathcal{O}$-module map
\[ c_{\mathcal{F}'} : \mathcal{I} \otimes _\mathcal {O} \mathcal{F} \longrightarrow \mathcal{K} \]
where $\mathcal{I} = \mathop{\mathrm{Ker}}(i^\sharp )$. Namely, given local sections $f$ of $\mathcal{I}$ and $s$ of $\mathcal{F}$ we set $c_{\mathcal{F}'}(f \otimes s) = fs'$ where $s'$ is a local section of $\mathcal{F}'$ lifting $s$.
Lemma 91.10.1. Let $i : (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}), \mathcal{O}) \to (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}), \mathcal{O}')$ be a first order thickening of ringed topoi. Assume given extensions
\[ 0 \to \mathcal{K} \to \mathcal{F}' \to \mathcal{F} \to 0 \quad \text{and}\quad 0 \to \mathcal{L} \to \mathcal{G}' \to \mathcal{G} \to 0 \]
as in (91.10.0.1) and maps $\varphi : \mathcal{F} \to \mathcal{G}$ and $\psi : \mathcal{K} \to \mathcal{L}$.
If there exists an $\mathcal{O}'$-module map $\varphi ' : \mathcal{F}' \to \mathcal{G}'$ compatible with $\varphi $ and $\psi $, then the diagram
\[ \xymatrix{ \mathcal{I} \otimes _\mathcal {O} \mathcal{F} \ar[r]_-{c_{\mathcal{F}'}} \ar[d]_{1 \otimes \varphi } & \mathcal{K} \ar[d]^\psi \\ \mathcal{I} \otimes _\mathcal {O} \mathcal{G} \ar[r]^-{c_{\mathcal{G}'}} & \mathcal{L} } \]
is commutative.
The set of $\mathcal{O}'$-module maps $\varphi ' : \mathcal{F}' \to \mathcal{G}'$ compatible with $\varphi $ and $\psi $ is, if nonempty, a principal homogeneous space under $\mathop{\mathrm{Hom}}\nolimits _\mathcal {O}(\mathcal{F}, \mathcal{L})$.
Proof.
Part (1) is immediate from the description of the maps. For (2), if $\varphi '$ and $\varphi ''$ are two maps $\mathcal{F}' \to \mathcal{G}'$ compatible with $\varphi $ and $\psi $, then $\varphi ' - \varphi ''$ factors as
\[ \mathcal{F}' \to \mathcal{F} \to \mathcal{L} \to \mathcal{G}' \]
The map in the middle comes from a unique element of $\mathop{\mathrm{Hom}}\nolimits _\mathcal {O}(\mathcal{F}, \mathcal{L})$ by Modules on Sites, Lemma 18.25.1. Conversely, given an element $\alpha $ of this group we can add the composition (as displayed above with $\alpha $ in the middle) to $\varphi '$. Some details omitted.
$\square$
Lemma 91.10.2. Let $i : (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}), \mathcal{O}) \to (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}), \mathcal{O}')$ be a first order thickening of ringed topoi. Assume given extensions
\[ 0 \to \mathcal{K} \to \mathcal{F}' \to \mathcal{F} \to 0 \quad \text{and}\quad 0 \to \mathcal{L} \to \mathcal{G}' \to \mathcal{G} \to 0 \]
as in (91.10.0.1) and maps $\varphi : \mathcal{F} \to \mathcal{G}$ and $\psi : \mathcal{K} \to \mathcal{L}$. Assume the diagram
\[ \xymatrix{ \mathcal{I} \otimes _\mathcal {O} \mathcal{F} \ar[r]_-{c_{\mathcal{F}'}} \ar[d]_{1 \otimes \varphi } & \mathcal{K} \ar[d]^\psi \\ \mathcal{I} \otimes _\mathcal {O} \mathcal{G} \ar[r]^-{c_{\mathcal{G}'}} & \mathcal{L} } \]
is commutative. Then there exists an element
\[ o(\varphi , \psi ) \in \mathop{\mathrm{Ext}}\nolimits ^1_\mathcal {O}(\mathcal{F}, \mathcal{L}) \]
whose vanishing is a necessary and sufficient condition for the existence of a map $\varphi ' : \mathcal{F}' \to \mathcal{G}'$ compatible with $\varphi $ and $\psi $.
Proof.
We can construct explicitly an extension
\[ 0 \to \mathcal{L} \to \mathcal{H} \to \mathcal{F} \to 0 \]
by taking $\mathcal{H}$ to be the cohomology of the complex
\[ \mathcal{K} \xrightarrow {1, - \psi } \mathcal{F}' \oplus \mathcal{G}' \xrightarrow {\varphi , 1} \mathcal{G} \]
in the middle (with obvious notation). A calculation with local sections using the assumption that the diagram of the lemma commutes shows that $\mathcal{H}$ is annihilated by $\mathcal{I}$. Hence $\mathcal{H}$ defines a class in
\[ \mathop{\mathrm{Ext}}\nolimits ^1_\mathcal {O}(\mathcal{F}, \mathcal{L}) \subset \mathop{\mathrm{Ext}}\nolimits ^1_{\mathcal{O}'}(\mathcal{F}, \mathcal{L}) \]
Finally, the class of $\mathcal{H}$ is the difference of the pushout of the extension $\mathcal{F}'$ via $\psi $ and the pullback of the extension $\mathcal{G}'$ via $\varphi $ (calculations omitted). Thus the vanishing of the class of $\mathcal{H}$ is equivalent to the existence of a commutative diagram
\[ \xymatrix{ 0 \ar[r] & \mathcal{K} \ar[r] \ar[d]_{\psi } & \mathcal{F}' \ar[r] \ar[d]_{\varphi '} & \mathcal{F} \ar[r] \ar[d]_\varphi & 0\\ 0 \ar[r] & \mathcal{L} \ar[r] & \mathcal{G}' \ar[r] & \mathcal{G} \ar[r] & 0 } \]
as desired.
$\square$
Lemma 91.10.3. Let $i : (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}), \mathcal{O}) \to (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}), \mathcal{O}')$ be a first order thickening of ringed topoi. Assume given $\mathcal{O}$-modules $\mathcal{F}$, $\mathcal{K}$ and an $\mathcal{O}$-linear map $c : \mathcal{I} \otimes _\mathcal {O} \mathcal{F} \to \mathcal{K}$. If there exists a sequence (91.10.0.1) with $c_{\mathcal{F}'} = c$ then the set of isomorphism classes of these extensions is principal homogeneous under $\mathop{\mathrm{Ext}}\nolimits ^1_\mathcal {O}(\mathcal{F}, \mathcal{K})$.
Proof.
Assume given extensions
\[ 0 \to \mathcal{K} \to \mathcal{F}'_1 \to \mathcal{F} \to 0 \quad \text{and}\quad 0 \to \mathcal{K} \to \mathcal{F}'_2 \to \mathcal{F} \to 0 \]
with $c_{\mathcal{F}'_1} = c_{\mathcal{F}'_2} = c$. Then the difference (in the extension group, see Homology, Section 12.6) is an extension
\[ 0 \to \mathcal{K} \to \mathcal{E} \to \mathcal{F} \to 0 \]
where $\mathcal{E}$ is annihilated by $\mathcal{I}$ (local computation omitted). Hence the sequence is an extension of $\mathcal{O}$-modules, see Modules on Sites, Lemma 18.25.1. Conversely, given such an extension $\mathcal{E}$ we can add the extension $\mathcal{E}$ to the $\mathcal{O}'$-extension $\mathcal{F}'$ without affecting the map $c_{\mathcal{F}'}$. Some details omitted.
$\square$
Lemma 91.10.4. Let $i : (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}), \mathcal{O}) \to (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}), \mathcal{O}')$ be a first order thickening of ringed topoi. Assume given $\mathcal{O}$-modules $\mathcal{F}$, $\mathcal{K}$ and an $\mathcal{O}$-linear map $c : \mathcal{I} \otimes _\mathcal {O} \mathcal{F} \to \mathcal{K}$. Then there exists an element
\[ o(\mathcal{F}, \mathcal{K}, c) \in \mathop{\mathrm{Ext}}\nolimits ^2_\mathcal {O}(\mathcal{F}, \mathcal{K}) \]
whose vanishing is a necessary and sufficient condition for the existence of a sequence (91.10.0.1) with $c_{\mathcal{F}'} = c$.
Proof.
We first show that if $\mathcal{K}$ is an injective $\mathcal{O}$-module, then there does exist a sequence (91.10.0.1) with $c_{\mathcal{F}'} = c$. To do this, choose a flat $\mathcal{O}'$-module $\mathcal{H}'$ and a surjection $\mathcal{H}' \to \mathcal{F}$ (Modules on Sites, Lemma 18.28.8). Let $\mathcal{J} \subset \mathcal{H}'$ be the kernel. Since $\mathcal{H}'$ is flat we have
\[ \mathcal{I} \otimes _{\mathcal{O}'} \mathcal{H}' = \mathcal{I}\mathcal{H}' \subset \mathcal{J} \subset \mathcal{H}' \]
Observe that the map
\[ \mathcal{I}\mathcal{H}' = \mathcal{I} \otimes _{\mathcal{O}'} \mathcal{H}' \longrightarrow \mathcal{I} \otimes _{\mathcal{O}'} \mathcal{F} = \mathcal{I} \otimes _\mathcal {O} \mathcal{F} \]
annihilates $\mathcal{I}\mathcal{J}$. Namely, if $f$ is a local section of $\mathcal{I}$ and $s$ is a local section of $\mathcal{H}$, then $fs$ is mapped to $f \otimes \overline{s}$ where $\overline{s}$ is the image of $s$ in $\mathcal{F}$. Thus we obtain
\[ \xymatrix{ \mathcal{I}\mathcal{H}'/\mathcal{I}\mathcal{J} \ar@{^{(}->}[r] \ar[d] & \mathcal{J}/\mathcal{I}\mathcal{J} \ar@{..>}[d]_\gamma \\ \mathcal{I} \otimes _\mathcal {O} \mathcal{F} \ar[r]^-c & \mathcal{K} } \]
a diagram of $\mathcal{O}$-modules. If $\mathcal{K}$ is injective as an $\mathcal{O}$-module, then we obtain the dotted arrow. Denote $\gamma ' : \mathcal{J} \to \mathcal{K}$ the composition of $\gamma $ with $\mathcal{J} \to \mathcal{J}/\mathcal{I}\mathcal{J}$. A local calculation shows the pushout
\[ \xymatrix{ 0 \ar[r] & \mathcal{J} \ar[r] \ar[d]_{\gamma '} & \mathcal{H}' \ar[r] \ar[d] & \mathcal{F} \ar[r] \ar@{=}[d] & 0 \\ 0 \ar[r] & \mathcal{K} \ar[r] & \mathcal{F}' \ar[r] & \mathcal{F} \ar[r] & 0 } \]
is a solution to the problem posed by the lemma.
General case. Choose an embedding $\mathcal{K} \subset \mathcal{K}'$ with $\mathcal{K}'$ an injective $\mathcal{O}$-module. Let $\mathcal{Q}$ be the quotient, so that we have an exact sequence
\[ 0 \to \mathcal{K} \to \mathcal{K}' \to \mathcal{Q} \to 0 \]
Denote $c' : \mathcal{I} \otimes _\mathcal {O} \mathcal{F} \to \mathcal{K}'$ be the composition. By the paragraph above there exists a sequence
\[ 0 \to \mathcal{K}' \to \mathcal{E}' \to \mathcal{F} \to 0 \]
as in (91.10.0.1) with $c_{\mathcal{E}'} = c'$. Note that $c'$ composed with the map $\mathcal{K}' \to \mathcal{Q}$ is zero, hence the pushout of $\mathcal{E}'$ by $\mathcal{K}' \to \mathcal{Q}$ is an extension
\[ 0 \to \mathcal{Q} \to \mathcal{D}' \to \mathcal{F} \to 0 \]
as in (91.10.0.1) with $c_{\mathcal{D}'} = 0$. This means exactly that $\mathcal{D}'$ is annihilated by $\mathcal{I}$, in other words, the $\mathcal{D}'$ is an extension of $\mathcal{O}$-modules, i.e., defines an element
\[ o(\mathcal{F}, \mathcal{K}, c) \in \mathop{\mathrm{Ext}}\nolimits ^1_\mathcal {O}(\mathcal{F}, \mathcal{Q}) = \mathop{\mathrm{Ext}}\nolimits ^2_\mathcal {O}(\mathcal{F}, \mathcal{K}) \]
(the equality holds by the long exact cohomology sequence associated to the exact sequence above and the vanishing of higher ext groups into the injective module $\mathcal{K}'$). If $o(\mathcal{F}, \mathcal{K}, c) = 0$, then we can choose a splitting $s : \mathcal{F} \to \mathcal{D}'$ and we can set
\[ \mathcal{F}' = \mathop{\mathrm{Ker}}(\mathcal{E}' \to \mathcal{D}'/s(\mathcal{F})) \]
so that we obtain the following diagram
\[ \xymatrix{ 0 \ar[r] & \mathcal{K} \ar[r] \ar[d] & \mathcal{F}' \ar[r] \ar[d] & \mathcal{F} \ar[r] \ar@{=}[d] & 0 \\ 0 \ar[r] & \mathcal{K}' \ar[r] & \mathcal{E}' \ar[r] & \mathcal{F} \ar[r] & 0 } \]
with exact rows which shows that $c_{\mathcal{F}'} = c$. Conversely, if $\mathcal{F}'$ exists, then the pushout of $\mathcal{F}'$ by the map $\mathcal{K} \to \mathcal{K}'$ is isomorphic to $\mathcal{E}'$ by Lemma 91.10.3 and the vanishing of higher ext groups into the injective module $\mathcal{K}'$. This gives a diagram as above, which implies that $\mathcal{D}'$ is split as an extension, i.e., the class $o(\mathcal{F}, \mathcal{K}, c)$ is zero.
$\square$
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