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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”.

4 votes

What is an immersed submanifold?

I think the answer to your final question is no, and more generally: countable unions of embedded submanifolds are precisely the images of (not-necessarily-injective) immersions. Sketch proof: A co …
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2 votes

Submanifolds of $\mathbb{R}^N$ whose local charts have uniformly bounded derivatives

Update: Everything I wrote before is fine, but here's a specific result, from Gilbarg-Trudinger Corollary 16.7 (2001 edition), as an idea of what's possible. Let $\Omega\subseteq \mathbb{R}^n$, and …
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9 votes

Derivative of the flow for ODEs on manifolds

As was pointed out by Deane Yang and Igor Khavkine in the comments, this feels like a fact that should be "looser" than Riemannian geometry. Indeed, as I will show below, your formula makes sense in …
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0 votes

Is this subset of matrices contractible inside the space of non-conformal matrices?

Yes, they are contractible in $\mathcal{NC}$, in fact even with $SL$ rather than $GL$. First simultaneously rotate $\mathcal{F}$, so the typical element is sent to $$ \begin{pmatrix} \cos a & -\sin a …
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13 votes
1 answer
622 views

Can a PDE constrain the degree of a $C^\infty$ map germ?

Let $\pi:E\to M$ be a smooth vector bundle over a smooth manifold, with $\text{rank}(E)=\text{dim}(M)$. For a section $\sigma$ of $E$ with a zero at $p\in M$, define the degree of the zero at $p$ to …
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