by Victor
(Moncton, Canada)

When you have 8 of the same number in any combination of 8 of the 9 by 9 squares, you can always put the ninth number using the scanning method.

For example, let’s say you have the number 4 in place in any 8 squares, you just use the scanning method to place the number 4 in the square where it was missing.

Try it, it’s always works, that’s a good help toward the solution.