# nLab derived category

### Context

#### Homological algebra

homological algebra

and

nonabelian homological algebra

diagram chasing

# Contents

## Idea

Derived categories were introduced by Jean-Louis Verdier in his thesis under the supervision of Alexandre Grothendieck. It was originally used to extend Serre duality? to a relative context. See Hartshorne?’s lecture notes “Residues and duality”.

The derived category $D\left(𝒜\right)$ of an abelian category $𝒜$ is the homotopy category of the (∞,1)-category of chain complexes in $𝒜$: the localization of the category of chain complexes ${\mathrm{Ch}}_{•}\left(𝒜\right)$ at the quasi-isomorphisms.

More in detail, associated to $𝒜$ is

The derived category $D\left(C\right)$ of $C$ is equivalently

In either case, this means that under the canonical localization functor

$Q:{\mathrm{Ch}}_{•}\left(𝒜\right)\to D\left(𝒜\right)$Q : Ch_\bullet(\mathcal{A}) \to D(\mathcal{A})

the quasi-isomorphisms of chain complexes become true isomorphisms and that $D\left(𝒜\right)$ is universal with respect to this property.

Hence the derived category is an approximation to the full simplicial localization of $K\left(𝒜\right)$. It is or can be equipped with several further properties and structure that give a more accurate approximation. Notably every derived category is a triangulated category, wich is a way of remembering the suspension and de-suspension operations on its objects – the suspension of chain complexes – hence its “stability”.

## Definition

Let $𝒜$ be an abelian category and $K\left(𝒜\right)$ its category of chain complexes modulo chain homotopy (the “homotopy category of chain complexes”).

Equip $K\left(C\right)$ with the structure of a homotopical category by declaring the weak equivalences to be the quasi-isomorphisms: those morphisms $f:V\to W$ which induce isomorphisms in homology, $H\left(f\right):H\left(V\right)\stackrel{\simeq }{\to }H\left(W\right)$.

###### Definition

The derived category $D\left(𝒜\right)$ is the homotopy category of $K\left(𝒜\right)$ with respect to these weak equivalences.

Analogously, for ${K}^{+,-,b}\left(𝒜\right)$ denoting the full subcategory on the chain complexes bounded above, bounded below, or bounded, respectively (see at category of chain complexes), one writes

${D}^{+,-,b}\left(𝒜\right)↪D\left(𝒜\right)$D^{+,-,b}(\mathcal{A}) \hookrightarrow D(\mathcal{A})

for the correspponding full subcategory of the derived category.

## Equivalent characterizations and constructions

There are various ways to construct or express the derived category more explicitly in terms of various special objects or morphisms in the category of chain complexes.

### In terms of localization at a null system

The “homotopy category of chain complexes$K\left(𝒜\right)$ is already a triangulated category. The derived category can be obtained as the construction of a homotopy category of a triangulated category with respect to a null system.

###### Definition

Let

$N\left(𝒜\right)\subset K\left(𝒜\right)$N(\mathcal{A}) \subset K(\mathcal{A})

and analogously

${N}^{+,-n,b}\left(𝒜\right)\subset {K}^{+,-,b}\left(𝒜\right)$N^{+,-n,b}(\mathcal{A}) \subset K^{+,-,b}(\mathcal{A})

be the full subcategory of $K\left(C\right)$ or on ${K}^{+,-,b}$, respectively, on those chain complexes $V$ whose chain homology vanishes in every degreey, ${H}_{•}\left(V\right)=0$.

###### Proposition

A chain map ${f}_{•}:{V}_{•}\to {W}_{•}$ is a quasi-isomorphism precisely there exists a distinguished triangle in $K\left(𝒜\right)$ of the form

$V\stackrel{f}{\to }W\to \mathrm{cone}\left(f\right)$V \stackrel{f}{\to} W \to cone(f)

with the mapping cone $\mathrm{cone}\left(f\right)\in N\left(𝒞\right)$.

###### Proposition

The derived category is equivalently the localization of $K\left(𝒜\right)$ at the null system $N\left(𝒜\right)$.

$D\left(𝒜\right)\simeq K\left(𝒜\right)/N\left(𝒜\right)\phantom{\rule{thinmathspace}{0ex}}.$D(\mathcal{A}) \simeq K(\mathcal{A})/N(\mathcal{A}) \,.

This perspective is discussed in (Kashiwara-Schapira, section 13) and (Schapira, section 6.2, 72).

### In terms of injective and projective resolutions

In the case that the underlying abelian category $𝒜$ has enough injectives or enough projectives, the hom sets in the derived category may equivalently be obtained as homotopy-classes of chain maps from projective resolutions to injective resolutions of chain complexes.

In view of the existence of the injective and projective model structure on chain complexes this is a special case of the general fact that homotopy categories of model categories may be obtained by forming homotopy classes of maps in the model category from cofibrant resolutions to fibrant resolutions. But here we spell out an direct discussion of this fact for chain complexes.

###### Definition

Write ${K}^{+}\left({ℐ}_{𝒜}\right)↪{K}^{+}\left(𝒜\right)$ for the full subcategory of the homotopy category of chain complexes bounded above on those that are degreewise injective objects.

Dually, let ${K}^{-}\left({𝒫}_{𝒜}\right)↪{K}^{-}\left(𝒜\right)$ for the full subcategory of the homotopy category of chain complexes bounded below on those that are degreewise projective objects.

###### Theorem

If $𝒜$ has enough injectives then the canonical functor

${K}^{+}\left({ℐ}_{𝒜}\right)\to {D}^{+}\left(𝒜\right)$K^+(\mathcal{I}_{\mathcal{A}}) \to D^+(\mathcal{A})

Dually, if $𝒜$ has enough projectives then the canonical functor

${K}^{-}\left({𝒫}_{𝒜}\right)\to {D}^{-}\left(𝒜\right)$K^-(\mathcal{P}_{\mathcal{A}}) \to D^-(\mathcal{A})

For instance (Schapira, cor. 7.3.2).

## References

The original reference is the thesis of Verdier:

A systematic discussion from the point of view of localization and homotopy theory is in section 13 of

and, similarly, in section 7 of

A pedagogical introduction is

A good survey of the more general topic of derived categories is

See in particular also the list of references given there.

Other lecture notes include

• Theo Bühler, An introduction to the derived category (pdf)

For a discussion in the context of (∞,1)-categories and in particular stable (∞,1)-categories see section 13, p. 53 of

For the applications of derived categories in algebraic geometry, see

Revised on May 28, 2013 20:49:57 by Adeel Ahmad Khan (160.45.40.186)