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Reorder order lemmas in modules
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aisejohan committed Jul 13, 2021
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Expand Up @@ -2488,6 +2488,39 @@ \section{Tensor product}
Omitted.
\end{proof}

\begin{lemma}
\label{lemma-tensor-commute-colimits}
Let $(X, \mathcal{O}_X)$ be a ringed space.
For any $\mathcal{O}_X$-module $\mathcal{F}$ the functor
$$
\textit{Mod}(\mathcal{O}_X) \longrightarrow \textit{Mod}(\mathcal{O}_X)
, \quad
\mathcal{G} \longmapsto \mathcal{F} \otimes_{\mathcal{O}_X} \mathcal{G}
$$
commutes with arbitrary colimits.
\end{lemma}

\begin{proof}
Let $I$ be a preordered set and let $\{\mathcal{G}_i\}$ be
a system over $I$. Set $\mathcal{G} = \colim_i \mathcal{G}_i$.
Recall that $\mathcal{G}$ is the sheaf associated to the presheaf
$\mathcal{G}' : U \mapsto \colim_i \mathcal{G}_i(U)$, see
Sheaves, Section \ref{sheaves-section-limits-sheaves}.
By
Lemma \ref{lemma-tensor-product-sheafification}
the tensor product $\mathcal{F} \otimes_{\mathcal{O}_X} \mathcal{G}$
is the sheafification of the presheaf
$$
U \longmapsto
\mathcal{F}(U) \otimes_{\mathcal{O}_X(U)} \colim_i \mathcal{G}_i(U) =
\colim_i \mathcal{F}(U) \otimes_{\mathcal{O}_X(U)} \mathcal{G}_i(U)
$$
where the equality sign is
Algebra, Lemma \ref{algebra-lemma-tensor-products-commute-with-limits}.
Hence the lemma follows from the description of colimits in
$\textit{Mod}(\mathcal{O}_X)$, see Lemma \ref{lemma-limits-colimits}.
\end{proof}

\begin{lemma}
\label{lemma-tensor-product-permanence}
Let $(X, \mathcal{O}_X)$ be a ringed space.
Expand Down Expand Up @@ -2541,8 +2574,10 @@ \section{Tensor product}
is exact. Using this we can prove (5). Namely, in this case there
exists locally such an exact sequence with $\mathcal{F}_i$, $i = 1, 2$
finite free. Hence the two terms
$\mathcal{F}_2 \otimes_{\mathcal{O}_X} \mathcal{G}$
are isomorphic to finite direct sums of $\mathcal{G}$.
$\mathcal{F}_2 \otimes_{\mathcal{O}_X} \mathcal{G}$ and
$\mathcal{F}_1 \otimes_{\mathcal{O}_X} \mathcal{G}$
are isomorphic to finite direct sums of $\mathcal{G}$ (for example
by Lemma \ref{lemma-tensor-commute-colimits}).
Since finite direct sums are coherent sheaves, these are coherent
and so is the cokernel of the map, see Lemma \ref{lemma-coherent-abelian}.

Expand Down Expand Up @@ -2570,39 +2605,6 @@ \section{Tensor product}
The proof of the other statements is omitted.
\end{proof}

\begin{lemma}
\label{lemma-tensor-commute-colimits}
Let $(X, \mathcal{O}_X)$ be a ringed space.
For any $\mathcal{O}_X$-module $\mathcal{F}$ the functor
$$
\textit{Mod}(\mathcal{O}_X) \longrightarrow \textit{Mod}(\mathcal{O}_X)
, \quad
\mathcal{G} \longmapsto \mathcal{F} \otimes_{\mathcal{O}_X} \mathcal{G}
$$
commutes with arbitrary colimits.
\end{lemma}

\begin{proof}
Let $I$ be a preordered set and let $\{\mathcal{G}_i\}$ be
a system over $I$. Set $\mathcal{G} = \colim_i \mathcal{G}_i$.
Recall that $\mathcal{G}$ is the sheaf associated to the presheaf
$\mathcal{G}' : U \mapsto \colim_i \mathcal{G}_i(U)$, see
Sheaves, Section \ref{sheaves-section-limits-sheaves}.
By
Lemma \ref{lemma-tensor-product-sheafification}
the tensor product $\mathcal{F} \otimes_{\mathcal{O}_X} \mathcal{G}$
is the sheafification of the presheaf
$$
U \longmapsto
\mathcal{F}(U) \otimes_{\mathcal{O}_X(U)} \colim_i \mathcal{G}_i(U) =
\colim_i \mathcal{F}(U) \otimes_{\mathcal{O}_X(U)} \mathcal{G}_i(U)
$$
where the equality sign is
Algebra, Lemma \ref{algebra-lemma-tensor-products-commute-with-limits}.
Hence the lemma follows from the description of colimits in
$\textit{Mod}(\mathcal{O}_X)$.
\end{proof}




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