Regular polyhedron surface tours
by Jan Kristian Haugland
Suppose a particle is moving around on the surface of a polyhedron. As long as it is in the interior of one of the faces, it moves in a
straight line. Any time it reaches an edge, it crosses it and keeps moving on the face on the other side at the same slope relative to that edge.
If it eventually reaches the initial position and direction without having been at a corner, we consider the tour as a planar graph with the
crossing points as vertices, and edges between consecutive crossing points. We call such a graph a regular surface tour for that polyhedron.
If the faces of the polyhedron are either all equilateral triangles or all squares, then there is a simple way to ensure that the tour is finite. On each face,
draw the same section of a triangular or square grid respectively, such that each corner coincides with a grid vertex. Let the particle move parallel to
the grid lines; just not on the lines themselves. We can characterize the grid by how many equal parts the sides are divided into by parallel grid lines.
The sides of triangles are divided into a, b and a+b parts, and the sides of squares are divided into a and b parts, for non-negative coprime integers a and b.
For details on the smallest graphs that come out of this for a specific polyhedron, click here: Tetrahedron Cube Octahedron Icosahedron
There are two infinite families of graphs that apparently occur as regular tours for the cube, the octahedron and the icosahedron simultaneously:
Type A has 2n(n+1) vertices and appears to be a regular tour for the cube with (a, b) = (4n+2, 1) and (3n+2, 3n+1),
for the octahedron with (a, b) = (3n+1, 1), and for the icosahedron with (a, b) = (3n+2, 3n+1) and (5n+2, 1).
Examples of graphs of type A:
Type B has 3n(n+1)/2 vertices and appears to be a regular tour for the cube with (a, b) = (2n+1, 1) and (2n+1, 2[n/2]+1),
for the octahedron with (a, b) = (n+1, n), and for the icosahedron with (a, b) = (2n+1, 2[n/2]+1) and (2n+[n/2]+1 ,1).
Examples of graphs of type B: