Now fill the grid column by column. For the first column, let aiþ1,1¼ai1þ1
(modulo 9) for all i¼1, 2, . . . 8. Do the same for the other eight columns to complete
the grid. With this procedure, it is clear that every column will contain exact those
nine numbers. It is also easy to see that a2j¼a1jþ1 (modulo 9) for all j¼1, 2, . . . 9.
Hence the second row, like the first row, will contain exact those nine numbers.
The same is also true for all other rows. Because a12¼a11þ3 (modulo 9) and
a13¼a12þ3 (modulo 9), we can also see that the first 3 by 3 sub-grid
contains exact nine numbers, and the same is true for all other 3 by 3 sub-grids.
Therefore, the completed 9 by 9 grid satisfies all the requirements (1)–(3) and forms
a solution.