 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 quasiisomorphisms in Kom(A), K(A) does so only for those that are quasiisomorphisms for a "good reason", namely actually having an inverse up to homotopy equivalence. Thus, K(A) is more understandable than D(A).
Contents
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 f − g = d_{B}h + hd_{A}.
This can be depicted as:
We also say that f and g are chain homotopic, or that f − g is nullhomotopic or homotopic to 0. It is clear from the definition that the maps of complexes which are nullhomotopic form a group.
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 clearer that this results in an additive category if one notes that this is the same as taking the quotient by the subgroup of nullhomotopic maps.
The following variants of the definition are also widely used: if one takes only boundedbelow (A^{n}=0 for n<<0), boundedabove (A^{n}=0 for n>>0), or bounded (A^{n}=0 for n>>0) complexes instead of unbounded ones, one speaks of the boundedbelow homotopy category etc. They are denoted by K^{+}(A), K^{}(A) and K^{b}(A), respectively.
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 quasiisomorphism. (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 (A[1])^{n} = A^{n + 1}),
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^{}(A) and K^{b}(A). Although triangles make sense in 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)). Less trivially, the rotation of a distinguished triangle is obviously not distinguished in Kom(A), but (less obviously so) 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 Hom_{HoC}(X,Y) = H^{0}Hom_{C}(X,Y). (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: SpringerVerlag, ISBN 9783540435839
 Weibel, Charles A. (1994), An introduction to homological algebra, Cambridge Studies in Advanced Mathematics, 38, Cambridge University Press, ISBN 9780521559874, OCLC 36131259, MR1269324
Categories:
Wikimedia Foundation. 2010.