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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).

32 votes
Accepted

Suspension of a topological space

It’s not true. The Poincare sphere $P$ is a manifold, and its suspension is not. But its double suspension is homeomorphic to $S^5$ by Cannon’s “Double Suspension Theorem”. I learned about this from …
Jeff Strom's user avatar
  • 12.5k
2 votes

Is there a crossing-free planar embedding of the 2-skeleton of the 6-simplex?

Here is a positive answer using connected sets whose boundary is not a Jordan curve. We want $m={n \choose k}$ connected sets, $S_1, S_2, \ldots , S_m\subseteq \mathbb{R}^2$. I'll want to associate …
domotorp's user avatar
  • 19.1k
2 votes

A conjecture on antipodes and Jordan curves on the sphere

Let's say $C$ divides $S^2$ into two disks, $D$ and $E$. Then, choosing a homeomorphism (fixing $C$) $h:D\to E$, we get an involution $t: S^2\to S^2$, which has degree $-1$. On the other hand, sinc …
Jeff Strom's user avatar
  • 12.5k
1 vote

"structure group" for fibration

My advisor (Sufian Husseini) wrote a book where something like this is done, based on the work in some of his first papers. Husseini, S. Y. The topology of classical groups and related topics. Gordon …
Jeff Strom's user avatar
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33 votes
Accepted

Is every real n-manifold isomorphic to a quotient of $\mathbb{R}^n$?

Hahn–Mazurkiewicz Theorem: Suppose $X$ is a nonempty Hausdorff topological space. Then the following are equivalent: there is a surjection $[0,1]\to X$, $X$ is compact, connected, locally connect …
Jeff Strom's user avatar
  • 12.5k
13 votes

What is the intuition behind the Freudenthal suspension theorem?

The suspension map $\Sigma: [A, X] \to [\Sigma A, \Sigma X]$ (of which yours is a special case) is adjoint to a map $[A, X] \to [A, \Omega\Sigma X]$ which is induced by the map $\sigma: X\to \Omega\S …
Jeff Strom's user avatar
  • 12.5k