# Hypercube of Duads

This picture by Greg Egan shows a hypercube with all vertices except the bottom labelled by **duads**, that is, 2-element subsets of a 6-element set. There are 15 duads, while the hypercube has 16 vertices.

This picture by Greg Egan shows a hypercube with all vertices except the bottom labelled by **duads**, that is, 2-element subsets of a 6-element set. There are 15 duads, while the hypercube has 16 vertices.

This picture shows the Tutte–Coxeter graph. This graph was discovered by the famous graph theorist William Thomas Tutte in 1947, but its remarkable properties were studied further by him and the geometer H. S. M. Coxeter in a pair of papers published in 1958.

This is the Heawood graph. This graph can be drawn on a torus with no edges crossing in such a way that it divides the torus into 7 hexagons, each pair of which shares an edge. In 1890, Percy John Heawood proved that for any map drawn on a torus, it takes at most 7 colors to ensure that no two countries sharing a common boundary have the same color. The Heawood graph proves that the number 7 is optimal.

This picture by Tilman Piesk shows the 14 Dyck words of length 8. A **Dyck word** is a balanced string of left and parentheses. In the picture, a left parenthesis is shown as upward-slanting line segment, and a right parenthesis as a downward-slanting one.

Suppose you have a set with 5 elements. There are 10 ways to choose a 2-element subset. Form a graph with these 10 choices as vertices, and with two vertices connected by an edge precisely when the corresponding subsets are disjoint. You get the graph shown here, called the **Petersen graph**.

This picture by Tilman Piesk shows the 15 partitions of a 4-element set, ordered by refinement. Finer partitions are connected to coarser ones by lines going down. In the finest partition, on top, each of the 4 elements is in its own subset. In the coarsest one, on bottom, all 4 elements are in the same subset.

The **Kepler problem** concerns a particle moving under the influence of gravity, like a planet moving around the Sun. Newton showed the orbit of such a particle is an ellipse, assuming it doesn’t fly off to infinity. There are many ways to prove this, but the most illuminating is to reparametrize time and think of the orbit as a circle in 4 dimensions. When the circle is projected down to 3-dimensional space, it becomes an ellipse. The animation in this post, created by Greg Egan, shows how this works.

This image by Greg Egan shows 5 ways to inscribe a regular tetrahedron in a regular dodecahedron. The union of all these is a nonconvex polyhedron called the compound of 5 tetrahedra, first described by Edmund Hess in 1876.

Here Greg Egan has drawn two regular dodecahedra, in red and blue. They share 8 corners—and these are the corners of a cube, shown in green. Adrian Ocneanu calls these **twin dodecahedra**, and has proved some fascinating results about them.

A **Schwarz triangle** is a spherical triangle that can be used to generate a tiling of a branched covering of the sphere by repeatedly reflecting this triangle across its edges. Sometimes we get an actual tiling of the sphere, but in general we get a branched covering, because the same point can lie in the interior of several triangles, and there may be branch points at the corners of the triangles.

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