The Stacks project

24.19 Localization and sheaves of differential graded modules

Let $(\mathcal{C}, \mathcal{O})$ be a ringed site. Let $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$ and denote

the corresponding localization morphism (Modules on Sites, Section 18.19). Below we will use the following fact: for $\mathcal{O}_ U$-modules $\mathcal{M}_ i$, $i = 1, 2$ and a $\mathcal{O}$-module $\mathcal{A}$ there is a canonical map

Namely, we have $j_!(\mathcal{M}_1 \otimes _{\mathcal{O}_ U} \mathcal{A}|_ U) = j_!\mathcal{M}_1 \otimes _\mathcal {O} \mathcal{A}$ by Modules on Sites, Lemma 18.27.9.

Let $\mathcal{A}$ be a differential graded $\mathcal{O}$-algebra. We will denote $\mathcal{A}_ U$ the restriction of $\mathcal{A}$ to $\mathcal{C}/U$, in other words, we have $\mathcal{A}_ U = j^*\mathcal{A} = j^{-1}\mathcal{A}$. In Section 24.18 we have constructed adjoint functors

with $j^*$ left adjoint to $j_*$. We claim there is in addition an exact functor

right adjoint to $j_*$. Namely, given a differential graded $\mathcal{A}_ U$-module $\mathcal{M}$ we define $j_!\mathcal{M}$ to be the graded $\mathcal{A}$-module constructed in Section 24.10 with differentials $j_!\text{d} : j_!\mathcal{M}^ n \to j_!\mathcal{M}^{n + 1}$. Given a homogeneous map $f : \mathcal{M} \to \mathcal{M}'$ of degree $n$ of differential graded $\mathcal{A}_ U$-modules, we obtain a homogeneous map $j_!f : j_!\mathcal{M} \to j_!\mathcal{M}'$ of degree $n$ of differential graded $\mathcal{A}$-modules. We omit the straightforward verification that this construction is compatible with differentials. Thus we obtain our functor.