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SUDOKU’S fiendishness has been tamed. We now know the minimum starting numbers, or clues, needed in a Sudoku puzzle grid for it to have only one solution.
The fewer clues there are in the 9 by 9 starting grid of a puzzle, the harder it is to complete, as there are more squares to fill. But too few clues, and the puzzle no longer has a unique solution. To understand why this is, imagine the extreme – a starting grid with just a single square filled in. This could clearly correspond to many different answers.
To find the minimum number of starting clues required, a team lead by Gary McGuire at University College Dublin, Ireland, turned to software that checks a completed Sudoku grid, looking for alternative puzzles buried within it. Nearly 50,000 single-solution 17-clue puzzles had already been found, so the researchers focused on finding a 16-clue one. It took the whole of 2011 to test all possibilities, and the team found none with a unique solution. This implies the minimum must be 17.
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