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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 …
Brendan McKay's user avatar
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 …
Brendan McKay's user avatar
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 …
Brendan McKay's user avatar
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.
Brendan McKay's user avatar
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 …
Brendan McKay's user avatar
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 …
Brendan McKay's user avatar
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 …
Brendan McKay's user avatar
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 …
Brendan McKay's user avatar