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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”.

14 votes
1 answer
1k views

Who proved that two homotopic embeddings of one surface in another are isotopic?

If $\Sigma_1$ and $\Sigma_2$ are two compact topological surfaces with boundary and $\phi, \psi : \Sigma_1 \hookrightarrow \Sigma_2$ are two orientation-preserving embeddings that are homotopic, then …
5 votes

Decomposition of a closed surface

It's the same proof. Take a topological pants decomposition as before, and look for a minimal-length representative on your given Riemannian metric. Then you invoke the theorem that if you have a simp …
Dylan Thurston's user avatar
3 votes
Accepted

Computing the hopf invariant (without integration or homology, as in Milnor) of the hopf map

If you have the Hopf link embedded in some standard way in $\mathbb{R}^3$, you can see the linking number as given by the degree of a map $S^1 \times S^1 \to S^2$ in a number of ways. For instance, t …
Dylan Thurston's user avatar