Lemma 13.9.15. Let $\mathcal{A}$ be an additive category. Let $A_1^\bullet \to A_2^\bullet \to \ldots \to A_ n^\bullet$ be a sequence of composable morphisms of complexes. There exists a commutative diagram

$\xymatrix{ A_1^\bullet \ar[r] & A_2^\bullet \ar[r] & \ldots \ar[r] & A_ n^\bullet \\ B_1^\bullet \ar[r] \ar[u] & B_2^\bullet \ar[r] \ar[u] & \ldots \ar[r] & B_ n^\bullet \ar[u] }$

such that each morphism $B_ i^\bullet \to B_{i + 1}^\bullet$ is a split injection and each $B_ i^\bullet \to A_ i^\bullet$ is a homotopy equivalence. Moreover, if all $A_ i^\bullet$ are in $K^{+}(\mathcal{A})$, $K^{-}(\mathcal{A})$, or $K^ b(\mathcal{A})$, then so are the $B_ i^\bullet$.

Proof. The case $n = 1$ is without content. Lemma 13.9.6 is the case $n = 2$. Suppose we have constructed the diagram except for $B_ n^\bullet$. Apply Lemma 13.9.6 to the composition $B_{n - 1}^\bullet \to A_{n - 1}^\bullet \to A_ n^\bullet$. The result is a factorization $B_{n - 1}^\bullet \to B_ n^\bullet \to A_ n^\bullet$ as desired. $\square$

Comment #297 by arp on

Heh since I'm nitpicking, it would look better if throughout the proof there were bullets on the complexes.

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