Things I should know

Random number: 68

Difference between homotopic and homeomorphic
Homeomorphic is stronger than homotopic. For instance, a deformation retract \(A\) of \(X\) is homotopic, but not necessarily homeomorphic. (Homeomorphic is literally isomorphic, you have a bijective "bicontinuous" map) Of course, deformation retract is a stronger condition than homotopic too.


Fundamental group of the real projective plane
Is NOT SIMPLE!! In fact: $$\pi(RP^2) \approx \langle a | a^2 \rangle$$ Link to understand why it's not simple that didn't really help; but the idea (I think) is that \(S^2\) is a covering space of \(RP^2\), so you can lift paths in \(RP^2\) into \(S^2\). The base point in \(RP^2\) corresponds to two antipodal points in \(S^2\). Non-trivial paths \(\gamma\) get lifted to a path in \(S^2\) that starts at one point and ends at the antipode. For some reason \(2\gamma\) is trivial when it gets lifted though...


Finding continuous maps
It's sometimes easier to take an indirect route. For instance; you'd think that there's no continuous surjective map from \(S^2\) to \(S^1\), but you'd be wrong. Just project \(S^2\) to a line, (i.e. just take the \(z\) component), and now wrap the line around the circle. Composition of continuous maps are continuous. Be indirect. Be holy.



Intuitive meaning of Short Exact Sequences
Here