24.12 Sheaves of differential graded algebras
This section is the analogue of Differential Graded Algebra, Section 22.3.
Definition 24.12.1. Let $(\mathcal{C}, \mathcal{O})$ be a ringed site. A sheaf of differential graded $\mathcal{O}$-algebras or a sheaf of differential graded algebras on $(\mathcal{C}, \mathcal{O})$ is a cochain complex $\mathcal{A}^\bullet $ of $\mathcal{O}$-modules endowed with $\mathcal{O}$-bilinear maps
\[ \mathcal{A}^ n \times \mathcal{A}^ m \to \mathcal{A}^{n + m},\quad (a, b) \longmapsto ab \]
called the multiplication maps with the following properties
multiplication is associative,
there is a global section $1$ of $\mathcal{A}^0$ which is a two-sided identity for multiplication,
for $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$, $a \in \mathcal{A}^ n(U)$, and $b \in \mathcal{A}^ m(U)$ we have
\[ \text{d}^{n + m}(ab) = \text{d}^ n(a)b + (-1)^ n a\text{d}^ m(b) \]
We often denote such a structure $(\mathcal{A}, \text{d})$. A homomorphism of differential graded $\mathcal{O}$-algebras from $(\mathcal{A}, \text{d})$ to $(\mathcal{B}, \text{d})$ is a map $f : \mathcal{A}^\bullet \to \mathcal{B}^\bullet $ of complexes of $\mathcal{O}$-modules compatible with the multiplication maps.
Given a differential graded $\mathcal{O}$-algebra $(\mathcal{A}, \text{d})$ and an object $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$ we use the notation
\[ \mathcal{A}(U) = \Gamma (U, \mathcal{A}) = \bigoplus \nolimits _{n \in \mathbf{Z}} \mathcal{A}^ n(U) \]
This is a differential graded $\mathcal{O}(U)$-algebra.
As much as possible, we will think of a differential graded $\mathcal{O}$-algebra $(\mathcal{A}, \text{d})$ as a graded $\mathcal{O}$-algebra $\mathcal{A}$ endowed with the operator $\text{d} : \mathcal{A} \to \mathcal{A}$ of degree $1$ (where $\mathcal{A}$ is viewed as a graded $\mathcal{O}$-module) satisfying the Leibniz rule given in the definition.
Part (3) follows immediately from the usual adjunction between $f^*$ and $f_*$ on sheaves of modules.
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