Sudoku • 6 November 2005
For a sudoku to have a unique solution, you have to start off with at least a certain number of boxes already filled in (“hints”).
What is the smallest possible number of hints for a sudoku with a unique solution?
Gordon Royle has collected 24620 examples of uniquely solvable sudoku with seventeen hints. No examples with only sixteen hints are known, but it has not been proven that none exists.