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The study of differentiable manifolds and differentiable maps. One fundamental problem is that of classifying manifolds up to diffeomorphism. Differential topology is what Poincaré understood as topology or “analysis situs”.

7 votes
2 answers
552 views

Is the union of a compact and the relatively compact components of its complementary in a ma...

I was thinking of a way to prove this and I realised that for my approach the lemma from the title would be useful, and it´s an interesting question on its own. Obviously it is true if the manifold is …
Saúl RM's user avatar
  • 10.6k
3 votes

Equidistant points on a compact Riemannian manifold

$K(M,g)$ depends on the metric, as shown by this question, which implies that we can change the metric of $\mathbb{R}^3$ so it has as many points pairwise at distance $1$ as we want.
Saúl RM's user avatar
  • 10.6k
40 votes
2 answers
2k views

Can the nth projective space be covered by n charts?

That is, is there an open cover of $\mathbb{R}P^n$ by $n$ sets homeomorphic to $\mathbb{R}^n$? I came up with this question a few years ago and I´ve thought about it from time to time, but I haven´t b …
Saúl RM's user avatar
  • 10.6k
14 votes
Accepted

Can a smooth manifold be realised as the image of a smooth function?

My comment turned answer: Any smooth $m$-manifold $M$ admits a complete Riemannian metric (for example, as this answer says, any manifold embeds into some Euclidean space as a closed subset by Whitney …
Saúl RM's user avatar
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