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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”.

2 votes

a small questions about hopf theorem

Suppose you have two maps $f$ and $g$ with common regular point p, and that each map has Brouwer degree k (and assume wlog $k>0$). Use $p_1,...,p_n$ to denote elements of $f^{-1}(p)$ and use $q_1,.. …
Jason DeVito - on hiatus's user avatar
15 votes
Accepted

Is an inextensible manifold necessarily compact?

Yes, $M$ must be compact. In fact, if $M$ is non-compact, it admits a non-surjective self embedding $f:M\rightarrow M$. When $n=1$, the only non-compact manifold is $\mathbb{R}$, which obviously admi …
Jason DeVito - on hiatus's user avatar
3 votes
Accepted

Smoothness of frame bundle of (global) orbifolds [reference request]

First, one can clearly assume $M$ is connected by simply applying the argument to each componenet of $M$. The key fact is a generalization of your argument for $M=\mathbb{R}^n$: that if $f:M\rightarr …
Jason DeVito - on hiatus's user avatar