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'Pole Star Sudoku' printed from http://nrich.maths.org/
by Henry Kwok
Rules of Pole Star Sudoku
This is a variation of Sudoku on a standard 9x9 grid which contains
a set of special clue-numbers.
These are small numbers placed always on the border lines between
selected pairs of neighbouring cells of the grid.
Each clue-number is the difference between the two numbers that
should be in the neighbouring cells just to the left and to the
right from that clue-number. For example, a clue-number 7 on the
border line means that one of the possible answers in the cell on
the left can be 9, 2, 8 or 1. If we choose 9 for the answer in the
cell, it means that the answer in the cell next to it on the right
is 2. If we choose the answer 2, it means that the answer on the
right is 9.
Not much information is there? However, fortunately for the solver,
you can use a starting digit (digit 9 in the last column) as the
Pole Star to guide you out of the "wilderness" of the puzzle.
Hence this Sudoku variant is named Pole Star Sudoku.
The remaining rules are as in a standard Sudoku: the object of the
puzzle is to fill in the whole 9x9 grid with numbers 1 through 9
(one number per cell) so that each horizontal line, each vertical
line, and each of the nine 3x3 squares (outlined with the bold
lines) must contain all the nine different numbers 1 through 9.
An article about this type of Sudoku can be found here