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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
1 answer
236 views

Reference request: Can iterated torus links be mutated?

I believe that most iterated torus links cannot be changed non-trivially by a Conway mutation, as follows. If you look at the JSJ decomposition of the double-branched cover, then each satellite torus …
Dylan Thurston's user avatar
6 votes

Tiling of genus 2 surface by 8 pentagons

Ian answered the second question as asked, but in case you meant to ask a different question: there is not always a symmetric tiling by regular polygons of the given type, even if those restrictions h …
Dylan Thurston's user avatar
18 votes
3 answers
630 views

Classification of tangles?

Has anybody done any work on making a classification of low-complexity tangles, analogous to the work for knots and links? I expect most of the small ones to be rational, and those that aren't rationa …
Dylan Thurston's user avatar
49 votes
2 answers
4k views

Can knot diagrams be monotonically simplified using under moves?

It is well known that knot diagrams cannot be monotonically simplified using Reidemeister moves. For instance, the Goeritz unknot cannot be directly simplified. On the other hand, there is a stronger …
Dylan Thurston's user avatar
10 votes
2 answers
238 views

What are the possible linking matrices of a quasi-positive link?

I was surprised recently to come across a 3-component link where the linking number of two of the components was negative. For a while I thought I had made a mistake, then I thought a little more and …
Dylan Thurston's user avatar
5 votes
1 answer
271 views

Is every triangulation of a Euclidean ball by convex tetrahedra shellable?

Suppose you are given a 3-ball $B$ in $\mathbb{R}^3$ that is bounded by a PL sphere, a triangulation $T$ of $B$ by Euclidean tetrahedra. Is that triangulation necessarily shellable? I know that if $ …
Dylan Thurston's user avatar
6 votes
Accepted

Thickening graphs to get honest actions

Every automorphism of $F_n$ can be realized by an automorphism of the 3-dimensional handlebody of genus $n$, obtained by attaching $n$ 1-handles to a 3-ball.
Dylan Thurston's user avatar
5 votes

Is there a version of Seiberg-Witten-Floer or Heegard-Floer homology for 3-manifolds with bo...

Andy Manion has already plugged our answer for Heegaard Floer homology. On the Seiberg-Witten side, not as much is known, but Tim Nguyen's thesis starts to attack the problem. However, there isn't a …
Dylan Thurston's user avatar
27 votes
Accepted

Usefulness of using TQFTs

All the answers so far have focused on 3 dimensions, but the answer is much more striking in 4 dimensions. Freedman's theorem tells you that classical homology invariants give you complete informatio …
Dylan Thurston's user avatar
12 votes
1 answer
713 views

Injectivity of the Dehn-Nielsen-Baer map?

If $S$ is a closed hyperbolic surface, is there an easy proof of the injectivity of the Dehn-Nielsen-Baer map from $\mathrm{Mod}(S)$ to $\mathrm{Out}(\pi_1(S))$, taking an element of the mapping class …
Dylan Thurston's user avatar
49 votes

Elegant proof that any closed, oriented 3-manifold is the boundary of some oriented 4-manifold?

I know of several different arguments. You can decide which one you think is most elegant... Rohlin's argument, which is actually quite geometric. You start with an immersion of the 3-manifold in $ …
Dylan Thurston's user avatar
8 votes
1 answer
392 views

To find a point in Teichmüller space or measured foliation, how many lengths of curves do yo...

To parametrize Teichmüller space, it suffices to measure the hyperbolic lengths of a finite number of curves. It is well-known that $9g-9$ curves suffice, by a standard pair-of-pants argument given in …
Dylan Thurston's user avatar
10 votes
2 answers
579 views

Is the Lisca-Matic bound (aka slice-Bennequin bound) strictly stronger than the Bennequin bo...

The Bennequin bound [1] says that, for a transverse knot (or later link) $K$ in $S^3$, $$\mathrm{sl}(K) \le - \chi(\Sigma)$$ for any Seifert surface $\Sigma$ for $K$, where $\mathrm{sl}$ is the self-l …
Dylan Thurston's user avatar
6 votes

Do the results of (1/n)-surgery determine the link?...

If the orginal link $U_1 \cup U_2$ was hyperbolic, the answer is yes. For large enough $n$, $S^3 \setminus K(n)$ will also be hyperbolic, and will approach $S^3 \setminus (U_1 \cup U_2)$ in the Grom …
Dylan Thurston's user avatar
7 votes
1 answer
310 views

Can a surface group act on a finite-valence simplicial tree?

Question. Let $S$ be a closed surface of genus $> 1$. Can $\pi_1(S)$ act faithfully and minimally on a simplicial tree of finite valence? Here "minimal" means that there is no invariant sub-tree. Thi …
Dylan Thurston's user avatar

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