The Stacks project

Definition 12.3.5. Given a pair of objects $x, y$ in a preadditive category $\mathcal{A}$, the direct sum $x \oplus y$ of $x$ and $y$ is the direct product $x \times y$ endowed with the morphisms $i, j, p, q$ as in Lemma 12.3.4 above.

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View Definition 12.3.5 as pdf