Definition 12.13.2. Let $\mathcal{A}$ be an additive category. We say a morphism $a : A_\bullet \to B_\bullet $ is a *homotopy equivalence* if there exists a morphism $b : B_\bullet \to A_\bullet $ such that there exists a homotopy between $a \circ b$ and $\text{id}_ A$ and there exists a homotopy between $b \circ a$ and $\text{id}_ B$. If there exists such a morphism between $A_\bullet $ and $B_\bullet $, then we say that $A_\bullet $ and $B_\bullet $ are *homotopy equivalent*.

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