07081300/Class notes for Tuesday, March 25

Typed Notes
The notes below are by the students and for the students. Hopefully they are useful, but they come with no guarantee of any kind.
First Hour
Definition
We define the CW chain complex via:
and the boundary maps via by
where is roughly the number of times that covers .
Let's make this precise.
For (not quite an embedding) this restricts to a map . Given let such that and the rest maps to the point .
Example:
If you just have the segment consisting of two endpoints and the line connecting them, and call this , then takes the two end points to and the rest (the open interval) gets mapped to . Hence, we get a circle.
We thus can now formally define
Theorem
is a chain complex; and
Examples:
1) for n>1
Hence and all the rest are zero. Hence, for p = 0 or n and is zero otherwise.
2) Consider the torus thought of as a square with the usual identifications and is the interior. Hence, is generated by the figure 8 with one loop labeled a and the other labeled b, and is generated by the entire torus.
Ie we get
Now,
but a takes the two endpoints of a (both p) and maps them to p. Neither point is mapped to . Hence,
Note: This ought to be checked from the definition of degree but was just stated in class
Now, = the degree of the map that takes the square to the figure 8...and hence is .
Hence the boundary map is zero at all places, so if n = 0 or 2, if n = 1 and is zero otherwise.
3) Consider the Klein bottle thought of as a square with the usual identifications. Under takes this to a circle with side labled b.
I.e.,
Where the sign may be negative. Or more eloquantly put: "2b or 2b, that is the question"
The kernal of is everything, so the homology is , and .
4) the "g holed torus" or "surface of genus g" is formed by the normal diagram with edges identified in sets of 4 such as
So, get
Therefore, if p = 2 or 0, if p = 1 and zero elsewhere.
5)
So,
The boundary map alternates between 0 and 2 where it is 2 for if i is even and 0 if it is odd.
Hence the homology alternates between for odd p and 0 for even p.
Second Hour
for ,
=
I.e.,
So, alternates between 0 (for odd p) and for even p and thus as this also as the homology. (and clearly trivial greater than p)