Prove a Möbius band is not equivalent to a band

algebraic-topologygeneral-topology

A band (or a disk with a hole in it) can be created by gluing two edges of a square in the same direction, while a Möbius band can be created by gluing two edges of a square in opposite directions. These two spaces are not homeomorphic (right?), but proving they are different seems much more difficult. They are homotopy equivalent, both connected — in general they seem to share the obvious properties.

What is the method of proving these two spaces are different?

I've worked my way through a good chunk of point-set topology but I don't have any foundations in algebraic topology. Does this problem require heavy algebraic topology to solve, or can I do it with point set topology?

Best Answer

Here is a result with a bit of point set.

Point set fact: if $X,Y$ are hausdorff and homeomorphic, then they have homeomorphic one point compactifications.

With this, you can show that the one point compactification of the mobius band is hoeomorphic to $\mathbb RP^2$. This can be seen explicitly by considering the punctured projective plane.

The one point compactification of the cyllinder is a sphere with two points identified.

You can either stop here if you are satisfied with orientability (or embeddings into $\mathbb R^3$.)

Otherwise, note that the latter space is homotopic to $S^2 \vee S^1$.

Now, you have the job of distinguishing $S^2 \vee S^1$ and $\mathbb RP^2$ up to homotopy is not so bad. For example, what are the possible maps $\pi_1(\mathbb RP^2) \to \pi_1(S^2 \vee S^1)$? Or said differently, there is a loop of order $2$ in $\mathbb RP^2$, is there anything like that in $S^2 \vee S^1$? So on, and so forth.