That is a nice instance, an optimization solver took 13 seconds to solve instead of the 0.2 seconds for the problem posted by the OP. Some people in combinator

That is a nice instance, an optimization solver took 13 seconds to solve instead of the 0.2 seconds for the problem posted by the OP. Some people in combinatorial optimization are addressing the problem of finding the hardest Sudoku problem. For an introduction of how the problem can be modeled, see: http://diuflx71.unifr.ch/lpl/lplmodels/puzzles/sudoku.pdf http://diuflx71.unifr.ch/lpl/lplmodels/puzzles/sudoku.pdf