Video (All of Section 19)
This section will constitute the proof of Theorem 15.1. As before, let be a topological space, and let and be open sets with . We name the inclusion maps as shown below:
This gives a sequence of chain maps as follows:
If this was a short exact sequence, then we could apply Theorem 17.2 to get the Mayer-Vietoris sequence. Unfortunately, however, this is not quite a short exact sequence: we will see that the first map is injective and the image of the first map is the kernel of the second one, but the second map is not surjective. We will need an extra step involving subdivision to deal with this issue.
Put
We also define and similarly for , and .
If then does not send the whole of into , but it may send some faces of into . Because of this, need not be closed under , so it need not be a subcomplex of . However, this will not matter for our immediate purposes.
We now note that
and all these unions involve disjoint sets. It follows that
Thus, in our earlier sequence, the map becomes the map
given by . Similarly, the map becomes the map
given by . From this it is clear that we have a short exact sequence
and an inclusion of chain complexes. By applying Theorem 17.2 to the short exact sequence, we get something which is essentially the Mayer-Vietoris sequence except that it involves instead of . To complete the construction of the Mayer-Vietoris sequence, we need to show that is actually the same as .
For any there exists such that .
We can easily reduce to the case where is a single map . Put and , so and are open in with . This means that is an open cover of , so Proposition 8.31 tells us that there is a Lebesgue number, say . The identity chain has diameter , so has diameter at most . If we choose large enough, then this diameter will be less than , and it will follow that every simplex involved in is either contained in or contained in . It follows that every simplex involved in the chain is either contained in or contained in , so as claimed. ∎
The homology of the quotient complex is zero.
First, note that the subdivision map sends to and to so it also sends the subcomplex to itself. We therefore have an induced map given by . Similarly, the chain homotopy induces a chain homotopy . We showed previously that on , and it follows that we have the same relation on . We therefore deduce from Proposition 14.7 that the map is the identity. Consider an element . This has the form for some . This in turn has the form for some . For sufficiently large we have , so , so . As is the identity this means that . Thus, we have as claimed. ∎
The inclusion induces an isomorphism .
This completes the construction of the Mayer-Vietoris sequence.