Lemma 84.14.4. In Situation 84.3.3 let $\mathcal{O}$ be a sheaf of rings on $\mathcal{C}_{total}$. Let $K$ be an object of $D(\mathcal{C}_{total})$. Set

\[ X_ n = (g_{n!}\mathcal{O}_ n) \otimes ^\mathbf {L}_\mathcal {O} K \quad \text{and}\quad Y_ n = (g_{n!}\mathcal{O}_ n \to \ldots \to g_{0!}\mathcal{O}_0)[-n] \otimes ^\mathbf {L}_\mathcal {O} K \]

as objects of $D(\mathcal{O})$ where the maps are as in Lemma 84.8.1. With the evident canonical maps $Y_ n \to X_ n$ and $Y_0 \to Y_1[1] \to Y_2[2] \to \ldots $ we have

the distinguished triangles $Y_ n \to X_ n \to Y_{n - 1} \to Y_ n[1]$ define a Postnikov system (Derived Categories, Definition 13.41.1) for $\ldots \to X_2 \to X_1 \to X_0$,

$K = \text{hocolim} Y_ n[n]$ in $D(\mathcal{O})$.

**Proof.**
First, if $K = \mathcal{O}$, then this is the construction of Derived Categories, Example 13.41.2 applied to the complex

\[ \ldots \to g_{2!}\mathcal{O}_2 \to g_{1!}\mathcal{O}_1 \to g_{0!}\mathcal{O}_0 \]

in $\textit{Ab}(\mathcal{C}_{total})$ combined with the fact that this complex represents $K = \mathcal{O}$ in $D(\mathcal{C}_{total})$ by Lemma 84.10.1. The general case follows from this, the fact that the exact functor $- \otimes ^\mathbf {L}_\mathcal {O} K$ sends Postnikov systems to Postnikov systems, and that $- \otimes ^\mathbf {L}_\mathcal {O} K$ commutes with homotopy colimits.
$\square$

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