proof of Brouwer fixed point theorem
Proof of the Brouwer fixed point theorem:
Assume that there does exist a map from with no fixed point. Then let be the following map: Start at , draw the ray going through and then let bethe first intersection
of that line with the sphere. This map is continuous
and well defined onlybecause fixes no point. Also, it is not hard to see that it must be the identity
on the boundarysphere. Thus we have a map , which is the identity on, that is, a retraction
. Now, if is the inclusionmap
, . Applying the reduced homology functor
, we find that, where indicates the induced map on homology
.
But, it is a well-known fact that (since is contractible), and that. Thus we have an isomorphism
of a non-zero group onto itselffactoring through a trivial group, which is clearly impossible. Thus we have a contradiction
,and no such map exists.