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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
Accepted

Does $\mathfrak{N}_4$ contain at least four distinct elements?

Thom proved that the unoriented bordism is a polynomial ring over $\mathbb{F}_2$ generated by elements $x_i$ with $i$ running over all numbers not of the form $2^k-1$. Thus, $\mathfrak{N}_4 = \mathbb{ …
Lennart Meier's user avatar
4 votes
Accepted

Do trivial homotopy groups imply existence of boundary preserving homotopies?

Let me first talk only about continuous maps. Your question becomes equivalent to asking whether every map from $\partial (M\times I) \to N$ (for $I$ the interval) can be extended to $M\times I$. This …
Lennart Meier's user avatar
12 votes

Conceptual proof of classification of surfaces?

This is more an extended comment than an answer to the question. The first thing to note is that there are different strenghts of the classification theorem for surfaces. Of course, there are the diff …
Lennart Meier's user avatar