Martin Henz Hoang-Minh Truong SudokuSat-A Tool for Analyzing Difficult Sudoku Puzzles. 25-35 2009 Tools and Applications with Artificial Intelligence http://dx.doi.org/10.1007/978-3-540-88069-1_3 series/sci/2009-166 db/series/sci/sci166.html#HenzT09