The Stacks project

Lemma 13.28.3. Let $\mathcal{A}$ be an abelian category. Let $\mathcal{I} \subset \mathop{\mathrm{Ob}}\nolimits (\mathcal{A})$ be a subset. Assume that every object of $\mathcal{A}$ is a subobject of an element of $\mathcal{I}$. Let $K^\bullet $ be a complex. There exists a commutative diagram

\[ \xymatrix{ \ldots \ar[r] & \tau _{\geq -2}K^\bullet \ar[r] \ar[d] & \tau _{\geq -1}K^\bullet \ar[d] \\ \ldots \ar[r] & I_2^\bullet \ar[r] & I_1^\bullet } \]

in the category of complexes such that

  1. the vertical arrows are quasi-isomorphisms,

  2. $I_ n^\bullet $ is a bounded below complex with terms in $\mathcal{I}$,

  3. the arrows $I_{n + 1}^\bullet \to I_ n^\bullet $ are termwise split surjections and $\mathop{\mathrm{Ker}}(I^ i_{n + 1} \to I^ i_ n)$ is an element of $\mathcal{I}$.

Proof. This lemma is dual to Lemma 13.28.1. $\square$


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