The following puzzle had been knocking around work (actually it was first given in our place in 1994): Given an n x n chessboard we wish to color the squares black and white with the property that for any subsquare (of size at least 2x2) the four corner squares are not all the same color. These are orthogonal squares (and not tilted ones). The original poser of the puzzle gave a number of 5x5 examples, and asked to either prove that it can be done for all n, or to show that it can't be done for some (and therefore all bigger) n0. I coded up this problem as a satisfiability problem and found a solution for 14x14, but, after 1 1/2 hours of computation the SAT solver (SatEliteGTI) reported that 15x15 is unsatisfiable. Has anyone else seen this puzzle? It would be much more satisfying if there were a proof that 15x15 was impossible by hand. Victor