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The Stacks project

Lemma 22.8.1. Let (A, \text{d}) be a differential graded algebra. Let 0 \to K \to L \to M \to 0 be an admissible short exact sequence of differential graded A-modules. The triangle

22.8.1.1
\begin{equation} \label{dga-equation-triangle-associated-to-admissible-ses} K \to L \to M \xrightarrow {\delta } K[1] \end{equation}

with \delta as in Lemma 22.7.2 is, up to canonical isomorphism in K(\text{Mod}_{(A, \text{d})}), independent of the choices made in Lemma 22.7.2.

Proof. Namely, let (s', \pi ') be a second choice of splittings as in Lemma 22.7.2. Then we claim that \delta and \delta ' are homotopic. Namely, write s' = s + \alpha \circ h and \pi ' = \pi + g \circ \beta for some unique homomorphisms of A-modules h : M \to K and g : M \to K of degree -1. Then g = -h and g is a homotopy between \delta and \delta '. The computations are done in the proof of Homology, Lemma 12.14.12. \square


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