Different minimal Sudokus can have a different number of clues. A minimal Sudoku is a Sudoku from which no clue can be removed leaving it a proper Sudoku. Ordinary Sudokus ( proper puzzles) have a unique solution. No exact results are known for Sudokus larger than the classical 9×9 grid, although there are estimates which are believed to be fairly accurate. Similar results are known for variants and smaller grids. There is a solvable puzzle with at most 21 clues for every solved grid, with the largest minimal puzzle found so far has 40 clues in the 81 cells. An ordinary puzzle with a unique solution must have at least 17 clues. There are 26 possible types of symmetry, but they can only be found in about 0.005% of all filled grids. įor classical Sudoku, the number of filled grids is 6,670,903,752,021,072,936,960 ( 6.671 ×10 21), which reduces to 5,472,730,538 essentially different solutions under the validity preserving transformations. Initial analysis was largely focused on enumerating solutions, with results first appearing in 2004. The analysis of Sudoku is generally divided between analyzing the properties of unsolved puzzles (such as the minimum possible number of given clues) and analyzing the properties of solved puzzles.
Mathematics can be used to study Sudoku puzzles to answer questions such as "How many filled Sudoku grids are there?", "What is the minimal number of clues in a valid puzzle?" and "In what ways can Sudoku grids be symmetric?" through the use of combinatorics and group theory.
A 24-clue automorphic Sudoku with translational symmetry
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