The Stacks project

\begin{equation*} \DeclareMathOperator\Coim{Coim} \DeclareMathOperator\Coker{Coker} \DeclareMathOperator\Ext{Ext} \DeclareMathOperator\Hom{Hom} \DeclareMathOperator\Im{Im} \DeclareMathOperator\Ker{Ker} \DeclareMathOperator\Mor{Mor} \DeclareMathOperator\Ob{Ob} \DeclareMathOperator\Sh{Sh} \DeclareMathOperator\SheafExt{\mathcal{E}\mathit{xt}} \DeclareMathOperator\SheafHom{\mathcal{H}\mathit{om}} \DeclareMathOperator\Spec{Spec} \newcommand\colim{\mathop{\mathrm{colim}}\nolimits} \newcommand\lim{\mathop{\mathrm{lim}}\nolimits} \newcommand\Qcoh{\mathit{Qcoh}} \newcommand\Sch{\mathit{Sch}} \newcommand\QCohstack{\mathcal{QC}\!\mathit{oh}} \newcommand\Cohstack{\mathcal{C}\!\mathit{oh}} \newcommand\Spacesstack{\mathcal{S}\!\mathit{paces}} \newcommand\Quotfunctor{\mathrm{Quot}} \newcommand\Hilbfunctor{\mathrm{Hilb}} \newcommand\Curvesstack{\mathcal{C}\!\mathit{urves}} \newcommand\Polarizedstack{\mathcal{P}\!\mathit{olarized}} \newcommand\Complexesstack{\mathcal{C}\!\mathit{omplexes}} \newcommand\Pic{\mathop{\mathrm{Pic}}\nolimits} \newcommand\Picardstack{\mathcal{P}\!\mathit{ic}} \newcommand\Picardfunctor{\mathrm{Pic}} \newcommand\Deformationcategory{\mathcal{D}\!\mathit{ef}} \end{equation*}

Remark 10.131.7. Suppose given a commutative diagram of rings

\[ \xymatrix{ B \ar[r] & B' \\ A \ar[u] \ar[r] & A' \ar[u] } \]

a $B$-module $M$, a $B'$-module $M'$, and a $B$-linear map $M \to M'$. Then we get a compatible system of module maps

\[ \xymatrix{ \ldots \ar[r] & P^2_{B'/A'}(M') \ar[r] & P^1_{B'/A'}(M') \ar[r] & P^0_{B'/A'}(M') \\ \ldots \ar[r] & P^2_{B/A}(M) \ar[r] \ar[u] & P^1_{B/A}(M) \ar[r] \ar[u] & P^0_{B/A}(M) \ar[u] } \]

These maps are compatible with further composition of maps of this type. The easiest way to see this is to use the description of the modules $P^ k_{B/A}(M)$ in terms of generators and relations in the proof of Lemma 10.131.3 but it can also be seen directly from the universal property of these modules. Moreover, these maps are compatible with the short exact sequences of Lemma 10.131.6.


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