Minimum Number of Starting Clues for Sudoku Is 17

Anecdotally, many Sudoku players know that the smallest possible number of starting clues an unsolved puzzle can contain at first is 17. In a new investigation, scientists took a closer look at this conjecture, in order to certify whether it was true or not.

After calculating all possible solution grids, one by one, for a 16-clue Sudoku puzzle, the researchers determined that no such puzzle exists. “Our search turned up no proper 16-clue puzzles, but had one existed, then we would have found it,” the team writes in the journal arXiv.

Conventionally, Sudoku puzzles are expected to have a single solution. If less than 17 clues are present at the beginning then, in one of the nine, 3x3 square containing nine digits each, there are always two numbers that can be interchanged.

This would automatically imply at least two solutions. If the scenario is repeated in more squares, then multiple solutions are possible. Therefore, in order to have a single solution, the minimum number of clues is 17.