What's the fewest starting numbers a Sudoku puzzle can have and still have one answer?
A standard 9x9 Sudoku needs enough starting numbers filled in to pin down a single unique solution; too few, and multiple different grids could satisfy the rules. Puzzle makers had long found working examples with 17 clues but never fewer. Was 17 actually the hard floor, or just the smallest anyone had happened to find?
Reveal the answer
In 2012, researchers Gary McGuire, Bastian Tugemann and Gilles Civario proved computationally that no valid Sudoku puzzle with only 16 clues can have a unique solution, confirming 17 really is the minimum. The proof required checking an enormous number of possible 16-clue grids, using roughly 7 million CPU hours on a supercomputing cluster.