Mirrored from Sudopedia, the Free Sudoku Reference Guide
Sukaku is one of the Sudoku Variations. It has the same rule as a standard Sudoku, but instead of givens, the clues are presented in the form of a pencilmark grid.
Here is an example:
.------------------------.------------------------.------------------------. | 14789 12349 389 | 136789 1236 12479 | 3458 123469 3789 | | 123469 15689 123578 | 124568 12346 12 | 45689 146 146789 | | 46789 2368 1235789| 1234567 234 23468 | 2678 169 3679 | :------------------------+------------------------+------------------------: | 36789 1459 4679 | 2489 2578 13478 | 34 1358 679 | | 25689 2356 379 | 1459 47 14679 | 246 358 58 | | 4689 34567 134789 | 23568 23789 345679 | 1235679 678 126 | :------------------------+------------------------+------------------------: | 24678 346789 12369 | 135679 13459 256789 | 579 168 258 | | 347 23579 3689 | 125679 1678 123459 | 135 1356 123458 | | 1567 245789 123478 | 48 12679 489 | 38 789 156789 | '------------------------'------------------------'------------------------'
The grid shows the possibilities from which to choose a digit for each cell. Where in a standard Sudoku a single placement eliminates multiple candidates, a Sukaku puzzle is more refined, since each individual candidate can be removed or left in the grid. As a result, the puzzle can be more complex than any standard Sudoku with givens.
The highest number of candidates currently known is 633. (Players's forum)
The toughness, measured in backdoor size, can be much higher than a standard Sudoku.