Origami Dodecahedra - Greg Egan

Origami Dodecahedra

There is a nice photograph of some interlocking origami dodecahedra created by Dirk Eisner on the website Mathematical Origami. But it’s hard to be sure how many dodecahedra the whole model contains, since some are hidden from view. This raises a puzzle: assuming the configuration is as symmetrical as possible, how many dodecahedra are there? Here you see Greg Egan’s answer to this puzzle—and to a much more challenging puzzle.

Icosahedron Illustrating Pentagon-Hexagon-Decagon Identity - Greg Egan

Pentagon-Hexagon-Decagon Identity

Suppose we inscribe a regular pentagon, a regular decagon, and a regular hexagon in circles of the same radius. If we denote the respective edge lengths of these polygons by $P$, $D$ and $H$, then these lengths obey $P^2=D^2+H^2$. So, the edges of a pentagon, decagon and hexagon of identical radii can fit together to form a right triangle! Recently Greg Egan gave a nice proof using the icosahedron.

Truncated Hypercube - Jos Leys, www.josleys.com

Truncated Hypercube

This is a truncated 4-dimensional cube, drawn in a curved style by Jos Leys. You can take an ordinary 3-dimensional cube, cut off its corners and get a truncated cube. Similarly, you can take a 4-dimensional cube, cut off its corners, and get a 4-dimensional uniform polytope with $2 \times 4 = 8$ truncated cubes as faces and $2^4 = 16$ tetrahedral faces! It’s called the truncated 4-cube.