# Homotopy category of chain complexes

In homological algebra in mathematics, the **homotopy category** *K(A)* of chain complexes in an additive category *A* is a framework for working with chain homotopies and homotopy equivalences. It lies intermediate between the category of chain complexes *Kom(A)* of *A* and the derived category *D(A)* of *A* when *A* is abelian; unlike the former it is a triangulated category, and unlike the latter its formation does not require that *A* is abelian. Philosophically, while *D(A)* makes isomorphisms of any maps of complexes that are quasi-isomorphisms in *Kom(A)*, *K(A)* does so only for those that are quasi-isomorphisms for a "good reason", namely actually having an inverse up to homotopy equivalence. Thus, *K(A)* is more understandable than *D(A)*.

## Definitions

Let *A* be an additive category. The homotopy category *K(A)* is based on the following definition: if we have complexes *A*, *B* and maps *f*, *g* from *A* to *B*, a **chain homotopy** from *f* to *g* is a collection of maps (*not* a map of complexes) such that

- or simply

This can be depicted as:

We also say that *f* and *g* are **chain homotopic**, or that is **null-homotopic** or **homotopic to 0**. It is clear from the definition that the maps of complexes which are null-homotopic form a group under addition.

The **homotopy category of chain complexes** *K(A)* is then defined as follows: its objects are the same as the objects of *Kom(A)*, namely chain complexes. Its morphisms are "maps of complexes modulo homotopy": that is, we define an equivalence relation

- if
*f*is homotopic to*g*

and define

to be the quotient by this relation. It is clear that this results in an additive category if one notes that this is the same as taking the quotient by the subgroup of null-homotopic maps.

The following variants of the definition are also widely used: if one takes only *bounded-below* (*A ^{n}=0 for n<<0*),

*bounded-above*(

*A*), or

^{n}=0 for n>>0*bounded*(

*A*) complexes instead of unbounded ones, one speaks of the

^{n}=0 for |n|>>0*bounded-below homotopy category*etc. They are denoted by

*K*,

^{+}(A)*K*and

^{−}(A)*K*, respectively.

^{b}(A)A morphism which is an isomorphism in *K(A)* is called a **homotopy equivalence**. In detail, this means there is another map , such that the two compositions are homotopic to the identities: and
.

The name "homotopy" comes from the fact that homotopic maps of topological spaces induce homotopic (in the above sense) maps of singular chains.

## Remarks

Two chain homotopic maps *f* and *g* induce the same maps on homology because *(f − g)* sends cycles to boundaries, which are zero in homology. In particular a homotopy equivalence is a quasi-isomorphism. (The converse is false in general.) This shows that there is a canonical functor to the derived category (if *A* is abelian).

## The triangulated structure

The *shift* *A[1]* of a complex *A* is the following complex

- (note that ),

where the differential is .

For the cone of a morphism *f* we take the mapping cone. There are natural maps

This diagram is called a *triangle*. The homotopy category *K(A)* is a triangulated category, if one defines distinguished triangles to be isomorphic (in *K(A)*, i.e. homotopy equivalent) to the triangles above, for arbitrary *A*, *B* and *f*. The same is true for the bounded variants *K ^{+}(A)*,

*K*and

^{−}(A)*K*. Although triangles make sense in

^{b}(A)*Kom(A)*as well, that category is not triangulated with respect to these distinguished triangles; for example,

is not distinguished since the cone of the identity map is not isomorphic to the complex 0 (however, the zero map is a homotopy equivalence, so that this triangle *is* distinguished in *K(A)*). Furthermore, the rotation of a distinguished triangle is obviously not distinguished in *Kom(A)*, but (less obviously) is distinguished in *K(A)*. See the references for details.

## Generalization

More generally, the homotopy category *Ho(C)* of a differential graded category *C* is defined to have the same objects as *C*, but morphisms are defined by
. (This boils down to the homotopy of chain complexes if *C* is the category of complexes whose morphisms do not have to respect the differentials). If *C* has cones and shifts in a suitable sense, then *Ho(C)* is a triangulated category, too.

## References

- Manin, Yuri Ivanovich; Gelfand, Sergei I. (2003),
*Methods of Homological Algebra*, Berlin, New York: Springer-Verlag, ISBN 978-3-540-43583-9 - Weibel, Charles A. (1994).
*An introduction to homological algebra*. Cambridge Studies in Advanced Mathematics.**38**. Cambridge University Press. ISBN 978-0-521-55987-4. OCLC 36131259. MR 1269324.