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Finite or discrete collections of geometric objects. Packings, tilings, polyhedra, polytopes, intersection, arrangements, rigidity.

22 votes
3 answers
821 views

Covering a hexagon

For $\epsilon > 0$ sufficiently small, can a regular hexagon with sides of length $1 + \epsilon$ be covered by seven equilateral triangles with sides of length $1$? Motivation: Conway and Soifer show …
Matthew Kahle's user avatar
8 votes

Is there a dense subset of the real plane with all pairwise distances rational?

Victor Klee and Stan Wagon write about this and other fun problems in their book: Old and New Unsolved Problems in Plane Geometry and Number Theory
Matthew Kahle's user avatar
2 votes

Erdos-Szekeres in high dimensions

I know that Morris and Soltan's survey article covers some higher-dimensional cases, but I don't remember if they have any bounds of the kind you're asking for. http://www.ams.org/journals/bull/2000- …
Matthew Kahle's user avatar
6 votes
1 answer
631 views

How many convex shapes can be made with the pieces of the Stomachion?

Tangrams are a well-known dissection of the square into seven convex polygons. One fun mathematical question is: how many convex rearrangements of the seven pieces are there? Answer: there are 12 mor …
Matthew Kahle's user avatar
7 votes
1 answer
355 views

Does the Hirsch conjecture hold for $n < 2d$?

The Hirsch conjecture asserts that the graph (i.e. $1$-skeleton) of a $d$-dimensional convex polytope with $n$ facets has diameter at most $n - d$. After being open for decades, Francisco Santos has …
Matthew Kahle's user avatar
39 votes
3 answers
2k views

Chromatic number of the hyperbolic plane

A notorious problem in combinatorics is the following: If we color $\mathbb{R}^2$ so that no pair of points at unit distance get the same color, what is the fewest number of colors required? This numb …
Matthew Kahle's user avatar
4 votes
Accepted

Average degree of contact graph for balls in a box

Torquato and Stillinger have a recent survey article that discusses some questions like this: Jammed hard-particle packings: From Kepler to Bernal and beyond They are particularly interested in rand …
Matthew Kahle's user avatar