Singular Homology - Functoriality

Functoriality

The construction above can be defined for any topological space, and is preserved by the action of continuous maps. This generality implies that singular homology theory can be recast in the language of category theory. In particular, the homology group can be understood to be a functor from the category of topological spaces Top to the category of abelian groups Ab.

Consider first that is a map from topological spaces to free abelian groups. This suggests that might be taken to be a functor, provided one can understand its action on the morphisms of Top. Now, the morphisms of Top are continuous functions, so if is a continuous map of topological spaces, it can be extended to a homomorphism of groups

by defining

where is a singular simplex, and is a singular n-chain, that is, an element of . This shows that is a functor

from the category of topological spaces to the category of abelian groups.

The boundary operator commutes with continuous maps, so that . This allows the entire chain complex to be treated as a functor. In particular, this shows that the map is a functor

from the category of topological spaces to the category of abelian groups. By the homotopy axiom, one has that is also a functor, called the homology functor, acting on hTop, the quotient homotopy category:

This distinguishes singular homology from other homology theories, wherein is still a functor, but is not necessarily defined on all of Top. In some sense, singular homology is the "largest" homology theory, in that every homology theory on a subcategory of Top agrees with singular homology on that subcategory. On the other hand, the singular homology does not have the cleanest categorical properties; such a cleanup motivates the development of other homology theories such as cellular homology.

More generally, the homology functor is defined axiomatically, as a functor on an abelian category, or, alternately, as a functor on chain complexes, satisfying axioms that require a boundary morphism that turns short exact sequences into long exact sequences. In the case of singular homology, the homology functor may be factored into two pieces, a topological piece and an algebraic piece. The topological piece is given by

which maps topological spaces as and continuous functions as . Here, then, is understood to be the singular chain functor, which maps topological spaces to the category of chain complexes Comp (or Kom). The category of chain complexes has chain complexes as its objects, and chain maps as its morphisms.

The second, algebraic part is the homology functor

which maps

and takes chain maps to maps of abelian groups. It is this homology functor that may be defined axiomatically, so that it stands on its own as a functor on the category of chain complexes.

Homotopy maps re-enter the picture by defining homotopically equivalent chain maps. Thus, one may define the quotient category hComp or K, the homotopy category of chain complexes.