[math-fun] Orthogonal adjacencies

Awhile ago, there was a thread about putting 1-9 in a grid so that all possible pairs were neighbors, when diagonal connections were allowed. It's a much, much harder problem when diagonal connections are not allowed.  If the given square is X'ed out in a 5x6 grid, the solution is unique, except for permutations of the digits.  All 45 possible number pairs each occur exactly once within the grid. 679226 123456 483753 46X711 995889 --Ed Pegg Jr www.mathpuzzle.com