Sudoku-X
Sudoku-X is one of the better known Sudoku Variations. It is similar to a standard Sudoku, but both diagonals must also contain digits 1 through 9.
Because the diagonals significantly reduce the number of Unavoidable Sets, the puzzle can contain less givens than a standard Sudoku. Also, the diagonals create opportunities for additional techniques, such as Skewed Fish (Triangular Connection, Crossover, Double Crossover, Finned Double Crossover, Skewed Swordfish), Multiple Crossover, Weak Link Square and Skewed XY-Wing.
The diagonal constraints are also used in several other variants and sizes. For example, the Killer variant can be combined with diagonal constraints to form Killer-X. A more extreme example would be Clueless-Shaolin-X.
Below is one easy Sudoku-X puzzle. It requires only singles and one application of Naked Pair.
Minimum Number of Givens
Ruud has a collection of Sudoku-X puzzles with as few as 12 givens. However, the actual minimum number of givens required for a valid Sudoku-X puzzle is not known.
