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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).

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
4 votes
2 answers
837 views

Fundamental polygons with infinite pairwise identifications

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
7 votes
1 answer
415 views

Free $\mathbb{Z}_2$-actions match at some point

I have in front of me a proof of this lemma: If $f$ and $g$ are free $\mathbb{Z}_2$-actions on $S^1$, then $f(x)=g(x)$ for some $x \in S^1$. A $\mathbb{Z}_2$-action on the unit circle $S^1$ is a …
Joseph O'Rourke's user avatar
8 votes
3 answers
1k views

Is the list of "known" 3D compact manifolds complete?

"it is an open question if the known compact manifolds in 3-D are complete." This is a quote from Eric Weisstein's CRC Concise Encyclopedia of Mathematics, Second Edition. 2010, p.480. (Google Bo …
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
1 vote
0 answers
156 views

Entangled helical knots

Consider a pair of disjoint, congruent helices $H_1$ and $H_2$ passing through one another in the following sense. (Caveat lector: This question is not of general interest! It is also long.) $H_1$ is …
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
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
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
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
65 votes
4 answers
4k views

Tying knots with reflecting lightrays

Let a lightray bounce around inside a cube whose faces are (internal) mirrors. If its slopes are rational, it will eventually form a cycle. For example, starting with a point $p_0$ in the interior of …
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
208 views

Fractional degree of a map?

Is there some natural notion of a fractional degree of a map? The degree of a map is a generalization of the winding number, and fractional winding numbers appear in the (mathematical physics) literat …
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

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