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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
0 answers
210 views

"A typical pair of two-dimensional surfaces in four dimensions will intersect at finitely ma...

The title quotes C.H. Taubes in the Princeton Companion to Mathematics (p.404). I found this surprising despite the natural lower-dimensional analog (a typical pair of loops in $\mathbb{R}^2$ will in …
Joseph O'Rourke's user avatar
1 vote
0 answers
217 views

Patterns in local winding number sequences

This is something of a followup to an earlier question Can any sequence of consecutive integers be realized as winding numbers?, whose answer is Yes. Now I would like to define a local winding number …
Joseph O'Rourke's user avatar
8 votes
1 answer
717 views

Can any sequence of consecutive integers be realized as winding numbers?

For a closed plane curve $C$, define its sequence of winding numbers to be the sorted list of the winding numbers of each of the distinct regions of the plane demarcated by $C$. For example, this curv …
Joseph O'Rourke's user avatar
24 votes
2 answers
2k views

Is every rational realized as the Euler characteristic of some manifold or orbifold?

Let me first ask the question for two-dimensional compact, connected manifolds and orbifolds. Then, if the answer is No, one can remove various conditions on the dimension, and allow non-compact examp …
Joseph O'Rourke's user avatar
49 votes
3 answers
8k views

Thurston's 24 questions: All settled?

Thurston's 1982 article on three-dimensional manifolds1 ends with $24$ "open questions":       $\cdots$ Two naive questions from an outsider: (1) Have all $24$ now been resolved? (2) If so, w …
Joseph O'Rourke's user avatar
41 votes
1 answer
6k views

Not all manifolds can be triangulated: In which dimensions?

I know that Ciprian Manolescu has settled the triangulation conjecture in the negative: Not all manifolds can be triangulated. I've only read secondary literature on this result, which did not detail …
Joseph O'Rourke's user avatar
15 votes
5 answers
3k views

Generalization of winding number to higher dimensions

Is there a natural geometric generalization of the winding number to higher dimensions? I know it primarily as an important and useful index for closed, plane curves (e.g., the Jordan Curve Theor …
Joseph O'Rourke's user avatar
2 votes
1 answer
101 views

Complexity of recognizing equivalent translation surfaces

"A translation surface is a union of polygons with pairs of parallel edges identified by translation, up to cut and paste equivalence." I take that succinct (and not fully precise) definition fro …
Joseph O'Rourke's user avatar
11 votes
4 answers
1k views

Distance between two knots

Are there some well-studied functions defining natural distance measures between two knots? One can imagine a function that counts, say, the minimum number of moves, each of which passes one strand of …
Joseph O'Rourke's user avatar
21 votes
1 answer
1k views

Homeomorphism historically: When did it reach its modern formulation?

Q. When did the notion of homeomorphism reach its modern formulation as a bicontinuous bijection, i.e., a continuous bijection between topological spaces whose inverse is also continuous? …
Joseph O'Rourke's user avatar
10 votes
3 answers
1k views

Which polygons have *simple* periodic billiard paths?

I know (or, rather, believe) that it remains unknown whether every polygon has a periodic billiard path. But Howard Masur proved in the 1980's that every rational polygon (vertex angles rational mult …
Joseph O'Rourke's user avatar
5 votes
0 answers
275 views

Is there a connection between |roots| $\rightarrow$ 1 and Gromov's waist theorem?

Recent questions showed that roots of a random polynomial tend to lie on the unit circle ("Why do roots of polynomials tend to have absolute value close to 1?"; "Distribution of roots of complex polyn …
Joseph O'Rourke's user avatar
5 votes
1 answer
482 views

Knot invariants in 3-manifolds that are not $\mathbb{R}^3$ or $S^3$ or $B^3$?

This is just a reference request; I have no sharp mathematical question. Inspired by the $(3+)$-year old MO question, In knot theory: Benefits of working in $S^3$ instead of $\mathbb{R}^3$?, I would …
Joseph O'Rourke's user avatar
10 votes
2 answers
2k views

Do all combinatorially distinct fundamental polygons correspond to surfaces?

The topology of a closed surface can be constructed by identifying edges of a fundamental polygon of an even number $2n$ of edges. Labeling the edges and using $\pm 1$ exponents to indicate direction, …
Joseph O'Rourke's user avatar
3 votes
1 answer
458 views

Surface curves equidistant from a simple closed geodesic

Let $S \subset \mathbb{R}^3$ be a surface embedded in $\mathbb{R}^3$, let's say (to keep it simple) of genus zero. Let $\gamma$ be a simple, closed, oriented geodesic on $S$. Because $\gamma$ is orien …
Joseph O'Rourke's user avatar

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