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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).

6 votes
Accepted

Space of halfspaces

The space is just $\mathbb{R} \times S^{n-1}$: each oriented hyperplane $H$ is identified by the normal unit vector $v$ together with the inner product of $v$ with an arbitrary $w \in H$.
Adam P. Goucher's user avatar
23 votes

What is the geometric shape of the Monster sporadic group?

In the penultimate chapter of Sphere Packings, Lattices and Groups, the authors define a $196884$-dimensional real vector space and a faithful representation of the Monster group on that space. Now, …
Adam P. Goucher's user avatar
10 votes

There is a 3-connected 5-regular simple $n$-vertex planar graph iff $n$ satisfies....?

There are no such graphs when $n$ is odd, by the handshaking lemma. Conversely, for all even $n \geq 224$, we claim such a graph exists. In particular, given two planar 5-regular graphs $G$, $H$ each …
Adam P. Goucher's user avatar
4 votes

Unusual symmetries of the Cayley-Menger determinant for the volume of tetrahedra

The following comment in the question intrigued me: In fact, it's possible to show that the linear symmetries of $\mathbb{R}^6$ that preserve the Cayley-Menger determinant form the Weyl group $D_6$, …
Adam P. Goucher's user avatar