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'Twin Chute-swapping Sudoku' printed from http://nrich.maths.org/
By Henry Kwok
Rules of Twin Chute-Swapping Sudokus
This Sudoku consists of a pair of linked standard Sudoku puzzles
each with some starting digits.
As usual, the object of this Sudoku is to fill in the whole of each
9x9 grid with digits 1 through 9 so that each row, each column and
each block contain all the digits 1 through 9.
Twin B is related to twin A in the following ways:
Given that twin A is the original puzzle, twin B is obtained by
swapping a horizontal/vertical chute or band of blocks with another
horizontal/vertical chute or band of blocks. By such
transformation, twin A and twin B are essentially the same or
equivalent Sudoku puzzle.
For example, fig 2 is created from fig 1 by shifting the first
chute of 3 blocks sideways to the right and the second chute of 3
blocks sideways to the left.
Similarly an equivalent puzzle (fig 3) can be created from fig 1 by
shifting the first chute of 3 blocks downwards and the second chute
of 3 blocks upwards.
The three puzzles are equivalent to one another. Equivalent puzzles
can also be created from fig 2 and fig 3 by similar