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Finite or discrete collections of geometric objects. Packings, tilings, polyhedra, polytopes, intersection, arrangements, rigidity.
3
votes
Erdős-Szekeres empty pseudoconvex $k$-gons
(Long comment, not an answer to the question posed.)
One way to generalise "general position" configurations of lines in the plane is to assign an orientation "clockwise" or "anticlockwise" to each s …
4
votes
Minimum number of common edges of triangulations
Note that it is possible to remove one or two points (maybe more) from Jukka's construction while retaining only 8 common lines. For example, removing the top left vertex of the array can be done like …
1
vote
Accepted
Bound for a sequence of vertices in a graph
Let $q$ be a prime power and let $P$ be a projective plane of order $q$. It has $q^2+q+1$ points and $q^2+q+1$ lines. Each point lies on $q+1$ lines, and each line has $q+1$ points. Each pair of lines …
2
votes
Accepted
Convex planar regions with all area bisectors having equal length
This paper has a reference to a positive answer to the first question.
3
votes
How different can the constituents of an Ehrhart quasi-polynomial be?
I'll complement Christian's answer with an example in the other direction. Consider the polytope of $8\times 8$ symmetric doubly-stochastic matrices with 0 diagonal. The period of the Ehrhart quasipol …
5
votes
Accepted
Do random triangulation edge-flips maintain randomness?
It disturbs uniformity. What you have is a Markov chain and it converges to a distribution given by the Perron eigenvector of the transition matrix. To preserve uniformity you need that eigenvector t …
2
votes
For which sets of $(n, m, k)$ does there exist an edge-labelling (using $k$ labels) on $K_n$...
This is a standard problem in design theory. A Steiner system $S(t, k, v)$ is a pair $(X, B)$, where $X$ is a $v$-element set and $B$ is a set of $k$-subsets of $X$, called blocks, with the property …
4
votes
Number of matrices with unit determinant and fixed sum of elements
(A comment rather than an answer.)
Here is a plot of $a_n/n^5$ (red) and $b_n/n^5$ (blue). It might not go far enough to show the asymptotic behaviour, but a possibility is that $a_n$ and $b_n$ are as …