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Topology of cell complexes and manifolds, classification of manifolds (e.g. smoothing, surgery), low dimensional topology (e.g. knot theory, invariants of 4-manifolds), embedding theory, combinatorial and PL topology, geometric group theory, infinite dimensional topology (e.g. Hilbert cube manifolds, theory of retracts).

62 votes
14 answers
12k views

What are some of the big open problems in 3-manifold theory?

From what I understand, the geometrization theorem and its proof helped to settle a lot of outstanding questions about the geometry and topology of 3-manifolds, but there still seems to be quite a lot …
10 votes

Topological Classification of Four-Manifolds

Suppose you can classify all open 4-manifolds. In particular you can classify all manifolds of the form $M^4 - pt$ where $M^4$ is a closed 4-manifold, and consequently you can classify all closed 4-m …
Paul Siegel's user avatar
  • 29.2k
0 votes
Accepted

Generalizations of Hopf-Rinow theorem

It seems that the answer to 1 is no: The Hopf-Rinow Theorem is false in infinite Dimensions. If you add the assumption that $M$ is a locally compact length space then the answer is "yes" by Theorem 2 …
Paul Siegel's user avatar
  • 29.2k
1 vote

Which automorphisms on $H_{1}(M^{3})$ are induced by homotopy equivalences?

Since you expressed interest in the hyperbolic case: it follows from the Mostow rigidity theorem and the Hurewicz theorem that an isomorphism $\phi \colon H_1(M) \to H_1(M)$ is induced by an isometry …
Paul Siegel's user avatar
  • 29.2k
13 votes
Accepted

Spin structures on $S^1$ and Spin cobordism

As Fabian pointed out in the comments, you have to be more careful about how you trivialize $SO(D^2)$. I'm going to use the standard coordinates $(x,y)$ on $\mathbb{R}^2$ (note that these are not glo …
Paul Siegel's user avatar
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8 votes

What should be taught in a 1st course on smooth manifolds?

I think there are two ways to approach a first course on manifolds: one can focus on either their geometry or their topology. If you want to focus on geometry, then I think Anton Petrunin's suggestio …
Paul Siegel's user avatar
  • 29.2k