Given a topological space , we previously defined a system of groups and homomorphisms . In Proposition 10.16 we showed that , and this allowed us to define the homology groups . In this section we will place all those constructions in a wider context, which will be useful for calculating .
A chain complex is a sequence of abelian groups and homomorphisms like
such that for all . We will often suppress the indices and just write instead of . We now put
The elements of are called cycles, and the elements of are called boundaries. As in Remark 12.13, the condition means that , so every boundary is a cycle. Because of this, it is legitimate to define
This is called the ’th homology group of the complex. Given a cycle , we write for the coset , and call this the homology class of . Thus is defined iff , and iff for some . The homomorphisms are called differentials.
We use the notation to refer to the whole chain complex as a single object. We also write for the whole system of groups , and similarly for and .
The content of Section 10 can now be expressed as follows: the groups form a chain complex (with differential ), and the homology groups of the space are defined to be the homology groups of the chain complex .
Note that the quotient is zero iff iff iff the sequence is exact. Thus, the size of the group can be regarded as a measure of how badly the chain complex fails to be exact at .
Consider a chain complex in which for , so the complex just has the form
Every differential either starts or ends at the zero group, so all differentials are zero. We find that and so . For we just have and and .
Now consider a chain complex in which for , so the complex just has the form
It is clear that for all , but may be nonzero. It follows that
We now consider a specific chain complex of the type discussed in Example 13.6, so for . We fix , and take and to be free abelian groups as follows:
These indices are supposed to be read modulo , so and and so on. We define by (so in particular ). We claim that the groups and are both isomorphic to .
The easiest way to see this is to introduce some alternative bases for these groups. We put
For example, when we have
It is easy to see that the list is a basis for over , and is a basis for . We also have
Most of the terms cancel, giving . For we can write this as . However, is the same as , so . In summary, we have
From this it is clear that and so . In other words, we have and .
We will typically express this answer by writing . The first group listed is , the second one is , and it is implicit that all subsequent groups are zero.
The method used above is fairly typical for calculation of the homology of small chain complexes. To calculate , we try to find bases for all the groups such that the differential sends every basis element to another basis element or to zero. We can then take the basis for and discard all pairs of basis elements with . The remaining elements will then give a basis for over , so in particular all the groups are free abelian groups.
There are some chain complexes for which the groups are not free abelian groups, and in those cases we will not be able to find a basis with properties as above. There is a more complicated algorithm that will still work in those cases, but we will not explain it here.
Let and be chain complexes. A chain map from to is a sequence of group homomorphisms such that for all we have . In other words, the following diagram must commute:
We will typically suppress all subscripts and superscripts and just write rather than .
We write for the sequence of identity maps . This is clearly a chain map from to itself. Now suppose we have chain maps . We define a chain map by the obvious rule ; this satisfies
as required. It is easy to see that this kind of composition is associative and that , so we have a category of chain complexes and chain maps.
Let be a chain map. Then
For all we have
For all we have
There is a well-defined map given by .
Moreover, these constructions give functors .
Suppose that , so . We then have with , so as required. (As mentioned in Remark 10.22, the notation is only meaningful if . Now we have checked that , we see that the notation used in (c) is valid.)
Suppose that , so for some . We then have as required.
From (a) and (b) we see that if then so so . It follows that there is an induced homomorphism given by
or in other words .
We can make into a functor by defining
As this is just a restriction of , it is clearly compatible with composition and identity morphisms, as required. We can make into a functor in the same way. Finally, suppose we have chain maps
For a homology class we have
From this it is clear that is also a functor. ∎
We now want to make homology into a functor from topological spaces to abelian groups. Let be a continuous map. Let be an element of the set , or equivalently, a continuous map . We define to be the composite function . Next, given an element we define . This extends the map linearly to give a homomorphism . We claim that this is a chain map, or in other words that whenever . As everything is extended linearly, it will be enough to prove this when , or equivalently . We then have
as required. It is clear that and , so we have defined a functor . We can compose this with the functor to get a functor which is traditionally also denoted by .
Suppose that is constant, say for all . We then claim that for all . Indeed, we can define by and , so . Thus, the map is the composite of the maps and . However, Proposition 10.23 tells us that , and the claim is clear from this.