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The game is played on a grid, most frequently a 9×9
grid made up of 3×3 subgrids. Some cells already contain
numbers. The goal is to fill in the empty cells, one number
in each, so that every column, row, and subgrids each contains
the numbers 1 through 9 once. Each number in the solution
occurs once only in each of three "directions",
hence "multiple isolations".
The attraction of the puzzle is that the completion rules
are simple, yet the overall task may be difficult. Published
puzzles are often ranked in terms of difficulty. This may
also be expressed in giving an estimated time to solve. Whilst
generally speaking the greater the number of givens the easier
the solution, the reverse is not necessarily true. The true
difficulty of the puzzle depends upon how easy it is to logically
determine subsequent numbers.
Sodoku is recommended by some teachers as a exercise in logical
reasoning. The puzzles are often free to use from published
sources and may also be custom-generated using software.
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