- how many finished 9x9 sudoku grids are there?
- how do you create the starting grids - is it simply backtracking from a finished square that fulfills the constraints?
- in connection with the previous, it should be noted that most starting grids i've seen have 180 degree rotational symmetry about a diagonal, sometimes 90 degree about both or reflectional about both, just like crossword grids
- is there a way to determine the 'difficulty' of a particular starting grid deterministically, or is it purely subjective?
if anyone has any ideas, i'd be interested...