The Stacks project

Lemma 13.11.5. Let $\mathcal{A}$ be an abelian category. Let $K^\bullet $ be a complex.

  1. If $H^ n(K^\bullet ) = 0$ for all $n \ll 0$, then there exists a quasi-isomorphism $K^\bullet \to L^\bullet $ with $L^\bullet $ bounded below.

  2. If $H^ n(K^\bullet ) = 0$ for all $n \gg 0$, then there exists a quasi-isomorphism $M^\bullet \to K^\bullet $ with $M^\bullet $ bounded above.

  3. If $H^ n(K^\bullet ) = 0$ for all $|n| \gg 0$, then there exists a commutative diagram of morphisms of complexes

    \[ \xymatrix{ K^\bullet \ar[r] & L^\bullet \\ M^\bullet \ar[u] \ar[r] & N^\bullet \ar[u] } \]

    where all the arrows are quasi-isomorphisms, $L^\bullet $ bounded below, $M^\bullet $ bounded above, and $N^\bullet $ a bounded complex.

Proof. Pick $a \ll 0 \ll b$ and set $M^\bullet = \tau _{\leq b}K^\bullet $, $L^\bullet = \tau _{\geq a}K^\bullet $, and $N^\bullet = \tau _{\leq b}L^\bullet = \tau _{\geq a}M^\bullet $. See Homology, Section 12.15 for the truncation functors. $\square$


Comments (2)

Comment #4314 by Linyuan Liu on

I think we should pick instead of . Besides, I don't think makes sense. We could take .


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