As I said before, the Sudoku (Su Doku) is not mathematical. It IS logical though. You can use other symbols to differentiate the blocks while the same rules apply. I believe that the least number of filled blocks necessary to complete a Sudoku is 17. Also there are Sudokus which have more that one possible solution, real hair pullers.

I've created Sudokus from scratch and it took me a little over an hour but I'm slow. Building them is like solving them. The thing to keep in mind is that the corner blocks suffice for that symbol in two edge directions. In any three, nine block sequence you will have paired symbols that repeat like an in-line 5-2; next block over different line, a 2-5 and so the last block of nine has to have a 2-5 or 5-2 combination. You can have the same occur with triplets but these are pretty easy to make/solve.

What is hard for me is knowing how much you can remove and still have the puzzle solvable. Since I got the little handheld electronic Sudoku generator I haven't really messed much with making my own. Lazy! I now, pretty much do them in bed as a means to going to sleep.