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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

Does exist a $\varepsilon$-tubular neighborhood of a smooth complex quasi-affine algebraic v...

If I understand the question correctly, no because $V \subset \mathbb{C}^2$ could be the union of $xy = 1$ with $x=0$. You can see from looking at the real solutions that it does not have a tubular n …
Greg Kuperberg's user avatar
5 votes

Can homologous submanifolds be connected by an immersed manifold with boundary?

First of all, it doesn't matter whether or not the map is smooth. If you find any continuous map, then it will have a smooth approximation. The other answers so far explain that the cobordism group …
Greg Kuperberg's user avatar
11 votes
Accepted

Morse Theory and Exotic Spheres

Milnor didn't explain the formula as much as maybe he should have, but the point is that the real part of a unit-length quaternion is invariant under both conjugation and inversion. Let $$r = ||u|| \ …
Greg Kuperberg's user avatar
23 votes
Accepted

Smooth bijection between non-diffeomorphic smooth manifolds?

Every smooth manifold has a smooth triangulation, which yields a pseudofunctor from the category of smooth manifolds to the category of PL manifolds. (There is no actual functor; that would be crazy. …
Greg Kuperberg's user avatar
4 votes

Which continuous functions are polynomials?

I'm going to go out on a limb and make a partial conjecture based on Tom Goodwillie's comment. A function $f:\mathbb{R}^n \to \mathbb{R}$ is topologically conjugate to a polynomial $p(x)$ only if it …
Greg Kuperberg's user avatar
38 votes
Accepted

Exotic differentiable structures on R^4?

I once heard Witten say that topology in 5 and higher dimensions "linearizes". What he meant by that is that the geometric topology of manifolds reduces to algebraic topology. Beginning with the Whi …
Greg Kuperberg's user avatar
47 votes

Can every manifold be given an analytic structure?

There is an amazing theorem of Morrey and Grauert that says that not only does every (paracompact) smooth manifold have a real analytic structure, the real analytic structure is unique. Using Whitney …
Greg Kuperberg's user avatar
17 votes
Accepted

Embeddings of $S^2$ in $\mathbb{CP}^2$

The conjecture that every $S^2 \subseteq \mathbb{C}P^2$ is standard if it is homologous to flat is implied by the smooth Poincaré conjecture in 4 dimensions. It also implies a special of smooth Poinc …
Greg Kuperberg's user avatar