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Homotopy theory, homological algebra, algebraic treatments of manifolds.

15 votes

Simply-connected rational homology spheres

Yes, every simply-connected rational homology $4$-sphere is topologically the $4$-sphere. Simply-connected closed topological $4$-manifolds are classified by their intersection form $Q_X:H^2(X;\Bbb Z …
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  • 773
5 votes
1 answer
230 views

$\pi_1$ of 4-manifolds that "look like" disk bundles

Let $X$ be a smooth compact oriented 4-manifold with $\partial X=L(p,1)$, $H_2(X;\Bbb Z)=\Bbb Z$, $H_3(X; \Bbb Z)=0$ and the induced map $\pi_1(L(p,1)) \to X$ surjective. What are the possibilities fo …
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  • 773
17 votes

Theorems that led to very successful research programs in Geometry and Topology

In 1982, Richard Hamilton proved that a smooth closed 3-manifold admitting a metric with strictly positive Ricci curvature is a spherical space form (in particular such a manifold which is simply conn …
3 votes

Is $TS^n$ diffeomorphic to an open subset of $\mathbb{R}^{2n}$

There are no sphere's with non-trivial normal bundle in that dimension. As far as I know, this is originally a theorem of Massey. See http://www.ams.org/journals/proc/1959-010-06/S0002-9939-1959-0109 …
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