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Discovered by Henry Kwok
Sudoku

Rules of Product Doubles Sudoku


Like the standard Sudoku, this puzzle has the basic rule:
Each column, each row and each box (3x3 subgrid) must have the numbers 1 through 9.

The puzzle can be solved with the help of small clue-numbers which are placed on the border lines between selected pairs of neighbouring cells of the grid.
Each clue-number is the product of two numbers that should be in the pair of the neighbouring cells just next to left and to right of that clue-number.

For example, clue-number 12 on the line between two neighbouring cells means that possible pairs of numbers for these cells can be from the following combinations: 2 and 6; 6 and 2; 3 and 4; or 4 and 3.

After finding the values of all the unknown digits, the puzzle is solved by the usual sudoku strategy.