There is no 16-Clue Sudoku: Solving the Sudoku Minimum Number of Clues Problem - Gary McGuire; Bastian Tugemann; Gilles Civario (CoRR, 2012)

There Is No 16-Clue Sudoku: Solving the Sudoku Minimum Number of Clues Problem via Hitting Set Enumeration. - Gary McGuire; Bastian Tugemann; Gilles Civario (Exp. Math., 2014)