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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).

7 votes

Is it possible that every edge in a 1-planar drawing with minimum number of crossings is cro...

It is not possible that in an optimal drawing of a 1-planar graph, every edge is crossed. Here is a proof. Suppose not and let $G$ be a smallest counterexample. I claim that $G$ is 2-connected. If …
Tony Huynh's user avatar
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4 votes
Accepted

Least cardinality of a set of points in the plane

As Boris Bukh points out, three points suffice, but I'd like to point out that your question is related to this MO question. Here is a summary of the information in the previous question. For the …
Tony Huynh's user avatar
  • 32.1k
19 votes
3 answers
995 views

Drawing planar graphs with integer edge lengths

It is well known that every planar graph has an embedding such that every edge is drawn as a straight line segment (Fáry's Theorem). Kemnitz and Harborth made the following stronger conjecture Conje …
Tony Huynh's user avatar
  • 32.1k
20 votes
2 answers
2k views

Simple curves on non-orientable surfaces.

Given an element in the (first) homology group of a surface, I would like to know if it can be represented as a simple closed curve. For orientable surfaces, this is well-known, but I wasn't able to …
Tony Huynh's user avatar
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17 votes

Applications of infinite graph theory

Here's a nice proof of the Cantor-Bernstein theorem in the language of infinite graphs. Theorem. Let $G$ be an infinite graph with bipartition $(A,B)$. If $G$ has a matching saturating $A$ and a m …
Tony Huynh's user avatar
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18 votes
2 answers
4k views

Turning pants inside-out (or backwards) while tied together

An entertaining topological party trick that I have seen performed is to turn your pants inside-out while having your feet tied together by a piece of string. For a demonstration, check out this vide …
Tony Huynh's user avatar
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