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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).

3 votes

Why almost every geodesic arc is generic?

The fact that the union of all simple geodesics has Hausdorff dimension 1 is a classic result of Joan Birman and Caroline Series Birman, Joan S., and Caroline Series. "Geodesics with bounded intersec …
Igor Rivin's user avatar
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3 votes

Is a generic closed orientable hyperbolic 3-manifold Haken?

Firstly, the random Heegard splitting model and the random fibering model are discussed at length in my preprint. (I believe the OP is familiar with it). I believe the question is asked there, though …
Igor Rivin's user avatar
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2 votes

Maximum order of MCG finite order elements

The answers to all the questions can be found in that fond of wisdom Farb-Margalit: Farb, Benson; Margalit, Dan, A primer on mapping class groups, Princeton Mathematical Series. Princeton, NJ: Princ …
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4 votes

Regarding the Thurston norm and the ways that a three-manifold can fiber over the circle

See Autumn Kent's answer to this question.
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0 votes

Action of U(1) on a sphere bundle, non-vanishing vector fields on odd-dimensional manifolds

A very closely related question is discussed here: nowhere vanishing vector field on a manifold
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22 votes
Accepted

Which closed orientable $4$-dimensional manifolds cannot be embedded in $6$-space?

This is true if and only if $X^4$ is spin and its signature vanishes. This is on p. 345 in Gompf/Stipsicz (4-manifolds and Kirby calculus), who cite Ruberman: Imbedding four-manifold and slicing links …
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2 votes

If a polyhedron is homeomorphic to a simplex, is it piecewise-linear homeomorphic?

I am not sure if this is what you are asking, but check out http://en.wikipedia.org/wiki/Exotic_R4 (note that in dimension four, PL is the same as smooth).
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3 votes

Sufficient conditions for a 3D tetrahedral complex to be homeomorphic to a 3D ball

First you check that your complex is a PL manifold with boundary (this is easy, the hardest part is checking that the links of interior vertices are 2-spheres (which is an euler characteristic argumen …
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8 votes

extension of surface homeomorphism

Really a long comment on @Agol's answer: One does not actually need the fact (which I was not aware of) to show what the OP wants from what @Agol says, since by Casson-Bleiler, any mapping class who …
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2 votes

extension of surface homeomorphism

I accidentally found this Rice University thesis (never published, it seems) which studies the question in some depth: Author Jamie Bradley Jorgensen Title Surface homeomorphisms that do not extend t …
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25 votes

Thurston's "tinker toy" problem

The result comes by way of Nash's theorem which states that every smooth manifold is a component of a real algebraic variety. Nash, John, Real algebraic manifolds, Ann. Math. (2) 56, 405-421 (1952). …
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5 votes
Accepted

Counterexample to high dimensional Nielsen realization problem

This is discussed at great length (with copious references) in Jonathan Block's and Shmuel Weinberger's paper. The title of the paper is the suggestive "On the generalized Nielsen realization problem" …
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8 votes

regular tiling of a surface of genus 2 by heptagons

A picture is in the comments to John Baez' Blog Post. EDIT For the second part of the question, no there is no obstruction, as long as the implied angle ($2\pi/v$) is smaller than the angle of a Eucli …
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6 votes

Determine if an $n$-dimensional mesh of simplices is a non-manifold

In all dimensions, something is a manifold if the link of every cell is a sphere. This, sadly, is undecidable if the dimension of your complex is at least five. It is decidable (but not quickly) for c …
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1 vote

homotopy groups of an orbifold

As a topological space, this is homotopy equivalent to $\mathbb{D}^3,$ so the homotopy groups are whatever they are for $\mathbb{D}^3.$ As an orbifold, the fundamental group is $\mathbb{PSL}(2, \mathb …
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