# Cubomino We can play domino with identical copes of a single cube by insisting that the cubes have matching colors at the faces where they touch. This is hard to convey in a perspective image like the one above that shows a 4x4x1 cubomino tiling, so I will switch to a 2D representation that shows each cube in central perspective from above. Here is the flattened version: As you can see, the single 3D cubomino can be represented by six 2D cubomino squares, which may be rotated: These are a subset of the tiles I used for the Compass game a while ago.

This new Cubomino puzzle is a simple example that teaches to analyze tilings by understanding them as boundary value problems. To see how this works, we first notice that to some extent the color of the lower and the left edge of a tile determines that tile and its rotation. It works for most color combinations, but there are exceptions. These occur precisely when the two chosen colors for bottom and left edge are antipodal, i.e. are either equal or are one of the three pairs of colors of two opposite faces of our cube: This observation extends to larger rectangles: No antipodal colors can occur both in the left edge and the bottom edge of a tiled rectangle. To see this, assume the contrary. The vertical edge on the left of the rectangle that has one of the antipodal colors (pale yellow below) determines a horizontal strip of tiles that have the antipodal colors as vertical edges, and the horizontal edge on the bottom of the rectangle with the second color from an antipodal pair (dark purple below) determines a vertical strip of tiles that have the antipodal colors as horizontal edges. These two strips meet in a tile that must have a boundary consisting just of antipodal colors, which is impossible. On the other hand, if the left and bottom edge have no antipodal colors in common, like in the example below,
there is always a unique tiling of a rectangle that has the two edges as its lower and left edge. The reconstruction process is easy: Each choice of a left and bottom edge color determines a tile and its rotation uniquely. We begin by placing the only possible square into the bottom left corner of our boundary, and work our way to the right and up.