X-Factor Sudoku

X-Factor Sudoku (aka Diagonal Sudoku and Sudoku X) is a standard 9×9 Sudoku with the two main diagonals in the grid highlighted. In addition to the usual rule that each row, each column and each 3×3 block must contain the numbers 1 to 9, so too must the two diagonals.

The additional rule ought to make the puzzle easier, but often this is not the case. As with many of the good sudoku variants, the additional help that the rule provides is cancelled out by giving fewer starter numbers. Adjusting the number of starters allows us to create a variety of difficulty levels.