What's the proper name for the following "4 dimensional donut"? Identifying opposite edges of a sheet of paper makes a torus, actually just the surface, either 0 or "1 cell" thick. Identifying 2 pairs of opposite sides of a 3-D rectangular solid (brick) makes a torus skin with some thickness, like the glaze of a donut. Now also identify the third pair of sides of the brick. This connects the inner surface of the glaze to the outer surface. That seems to require a fourth dimension, so I call this a "4 dimensional donut". But what is its correct name? I'm still working on domino-nets. The smallest "4-D donut" that might have solutions is 4x5x5, for donimos with 1 to 25 pips (no double dominos). This is probably too large for me to either find a solution or to prove there are none, unfortunately. Thanks, -- Mike Beeler