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Suppose we have a morphism between short exact sequences of chain complexes valued in an abelian category $\mathcal{D}$, where the left and middle vertical arrows are chain homotopy equivalences $\require{AMScd}$

\begin{CD} 0 @>>> A @>>> B @>>> C @>>> 0\\ @. @V \simeq VV @V \simeq VV @V f VV @. \\ 0 @>>> A' @>>> B' @>>> C' @>>> 0 \end{CD}

The five lemma shows that $f$ is a quasi-isomorphism. Replacing the first two vertical maps with their chain homotopy inverses (assume that the resulting diagram also commutes) induces a weak inverse for $f$, that is, a morphism $g$ such that $H_*(f)$ and $H_*(g)$ are mutually inverse. Is it also true that this weak inverse is a chain homotopy inverse? If not true in general, are there conditions on $\mathcal{D}$ or the chain complexes that would make this true?

Edit: It seems straightforward to show via a diagram chase that this is true if the chain homotopies themselves commute in the diagram. That is, if \begin{CD} 0 @>>> A @> i >> B @>>> C @>>> 0\\ @. @V \simeq V aV @V \simeq V bV @V gf VV @. \\ 0 @>>> A @> i >> B @>>> C @>>> 0 \end{CD} commutes, and $P: a \Rightarrow \mathbf{1}$ and $Q : b \Rightarrow \mathbf{1}$ are chain homotopies such that $Qi = iP$, then $gf$ is chain homotopy equivalent to the identity on $C$. If we assume a similar commutativity condition for $P', Q'$ (the apparent chain homotopies in the other direction) then the result holds. I'm wondering if these assumptions can be weakened, or if they depend on each other somehow.

abhi01nat
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    If you replace the first two vertical maps with their chain homotopy inverses, there’s no reason to expect that the square you get on the left will commute (except up to chain homotopy), and so in general it will not induce a weak inverse $g$ to $f$. – Jeremy Rickard Apr 20 '23 at 17:22
  • @JeremyRickard fixed, thanks! – abhi01nat Apr 21 '23 at 10:43

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