and
nonabelian homological algebra
Let $\mathcal{A}$ be an additive category.
Recall the notion of chain complex, of chain map between chain complexes and of chain homotopy between chain maps in $\mathcal{A}$.
Call a chain complex $C_\bullet$
bounded below if there is $k \in \mathbb{N}$ such that $C_{n \leq k} = 0$;
bounded above if there is $k \in \mathbb{N}$ such that $C_{n \geq k} = 0$;
bounded if it is bounded below and bounded above. We have
Write $Ch_\bullet(\mathcal{A})$ for the category whose objects are chain complexes in $\mathcal{A}$ and whose morphisms are chain maps between these.
This is the category of chain complexes in $\mathcal{A}$.
Several variants of this category are of relevance.
Write $Ch_\bullet^{+,-,b}(\mathcal{A}) \hookrightarrow Ch_\bullet(\mathcal{A})$ for the full subcategory on the chain complexes which are, respectively, bounded above, bounded below or bounded.
Write $K(\mathcal{A})$ for the category obtained from $Ch_\bullet(\mathcal{A})$ by identifying homotopic chain maps.
Accordingly $K^{+,-,b}(\mathcal{A}) \hookrightarrow K(\mathcal{A})$ denotes the full subcategory on the chain complexes bounded above, bounded below or bounded, respectively.
This is sometimes called the homotopy category of chain complexes. But see the warning on terminology there, as this term is also appropriate for the category in the following remark.
If $\mathcal{A}$ is moreover an abelian category, then there is also the derived category $D(\mathcal{A})$, obtained from $Ch_\bullet(\mathcal{A})$ or $K(\mathcal{A})$ by universally inverting all quasi-isomorphisms. See at derived category for more on this.
For $\mathcal{A}$ an abelian category also the category of chain complexes $Ch_\bullet(\mathcal{A})$ is again an abelian category.
We discuss the ingredients that go into this statement.
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For $f : C_\bullet \to D_\bullet$ a chain map,
A sequence of chain complexes $0 \to A_\bullet \to B_\bullet \to C_\bullet \to 0$ is a short exact sequence in $Ch_\bullet(\mathcal{A})$ precisely if each component $0 \to A_n \to B_n \to C_n \to 0$ is a short exact sequence in $\mathcal{A}$.
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Equipped with the standard tensor product of chain complexes $\otimes$ the category of chain complexes is a monoidal category $(Ch_\bullet(R Mod), \otimes)$. The unit object is the chain complex concentrated in degree 0 on the tensor unit $R$ of $R Mod$.
In fact $(Ch_\bullet, \otimes)$ is a closed monoidal category, the internal hom is the standard internal hom of chain complexes.
A basic introduction is in chapter 1 of