I posted previously about my talk at Gathering for Gardner 12 on colorings of pentomino tilings ^{[1]}. One unexpected consequence of that is that my work has now been cited in a very prestigious… um… coloring book. Alex Bellos and Edmund Harriss previously collaborated on Patterns of the Universe ^{[2]}, a mathematical coloring book for adults, and were looking for material for the sequel ^{[3]}. They were attending G4G12, and saw my talk, and thought that they had found some. They wanted to use something like the strict complete combination 3,4-coloring of the pentomino tiling that I showed, but for the purpose of a coloring book page, they needed something with more shapes to color. Could I come up with such?

It seemed to me that the problem called for a pentomino tiling of a torus, which they could use as a wallpaper-like pattern, repeated as many times as they needed. The choice of the particular torus to use is a matter of taste, but I thought it would be nice to maximize the minimum distance between two images of the same point. (I haven’t proven that I succeeded, but it’s close.) In coding the solver for this, I used a shortcut: instead of directly checking whether a given tiling had a coloring of the correct type, I checked whether each pentomino bordered exactly six others. This turns out to be a necessary condition, but not a sufficient one, so I manually checked a few such tilings until I found one that worked. This is the pattern that appears, in user-colorable form, in Visions of The Universe ^{[3]} by Bellos and Harriss.

The hexiamonds were the other obvious set of 12 polyforms to try to tile with this coloring scheme. Here, there is one torus with maximal symmetry. Amazingly, my solver found just two tilings where every piece bordered 6 others, of which exactly one had the right coloring properties. Recall that the solution for the pentominoes on the 6×10 rectangle was also unique. It seems incredible to me that this problem type has yielded two instances that were so finely balanced as to be solvable, but only by the barest of margins.