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'Twin Line-swapping Sudoku' printed from http://nrich.maths.org/
By Henry Kwok
Here is a
Word document with the Twin Sudoku that you can download.
Rules of Twin Line-swapping Sudoku
This Sudoku variant consists of a pair of linked standard
Sudoku puzzles each with some starting digits.
As usual, the object of this Sudoku variant 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
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 the rows/columns within a chute/band of blocks. By such
transformation, twin A and twin B are essentially the same Sudoku
For example, figure 2 is obtained from figure 1 below by:
swapping row 1 and row 3 within the top horizontal chute/band of
blocks, swapping row 5 and row 6 within the middle horizontal
chute/band of blocks, swapping row 7 and row 8 within the bottom
horizontal chute/band of blocks.
Similarly an equivalent puzzle (figure 3) can be obtained from
figure 1 by: swapping column 1 and column 3 within the left
vertical chute/band of blocks, swapping column 5 and column 6
within the middle vertical chute/band of blocks, swapping column 7
and column 8 within the right vertical chute/band of blocks.
The three puzzles are equivalent to one another. Equivalent puzzles
can also be created from figure 2 and figure 3 by similar