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Enumerative combinatorics, graph theory, order theory, posets, matroids, designs and other discrete structures. It also includes algebraic, analytic and probabilistic combinatorics.

8 votes

A result from Peter McMullen's thesis

This is theorem 4C6 of Peter McMullen's thesis, "On the Combinatorial Structure of Convex Polytopes", on page 73: 4C6. Theorem. A $d$-polytope $P$ is regular if and only if for each $j = 0, \dots, …
Nick Matteo's user avatar
27 votes
1 answer
2k views

Solutions to $\binom{n}{5} = 2 \binom{m}{5}$

In Finite Mathematics by Lial et al. (10th ed.), problem 8.3.34 says: On National Public Radio, the Weekend Edition program posed the following probability problem: Given a certain number of bal …
Nick Matteo's user avatar
4 votes

Is there any edge- but not vertex-transitive polytope in $d\ge 4$ dimensions?

If you consider a tiling of 3-space to be a 4-dimensional polytope, then the Rhombic dodecahedral honeycomb would work. Other possibilities are limited by the potential 3-faces. Because every edge h …
Nick Matteo's user avatar