I read in the lecture notes of my course on manifolds (undergraduate) a little side-note that stated that every manifold with dimension less then 4 can be equipped with a differentiable structure, but without proof.
I was interested in this statement, so I started to look on the internet and in the library for a proof, but I couldn't find it anywhere!
At first I tried to prove it myself, but then some websites stated that is was a very difficult proof.
Could you help me with the proof? I honestly have to say I don't have a clue where to start. Maybe first embed it in euclidean space?
If you also think the proof is way to difficult for an undergraduate, could you then please help me find a place where I can read it?
Thank you very much,