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'Product Doubles Sudoku' printed from http://nrich.maths.org/
Discovered by Henry Kwok
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
After finding the values of all the unknown digits, the puzzle is
solved by the usual sudoku strategy.