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'Difference Sudoku' printed from http://nrich.maths.org/
By Henry Kwok
Rules of Difference Sudoku
Like the standard sudoku, the object of the puzzle is to fill in
the whole $9 \times 9$ grid with numbers $1$ through $9$ so that
each row, each column, and each of the nine $3 \times 3$ squares
must contain all the nine different numbers.
There are special clue-numbers placed on the border lines between
selected pairs of adjacent squares of the grid. Each clue-number is
the difference between the two numbers that should be in the
adjacent squares just next to left & right from that
For example, a clue-number $7$ on the border line between two
adjacent squares means that possible pairs of numbers for these
squares must be from the following combinations: $1$ and $8$; $2$
and $9$; $8$ and $1$; or $9$ and $2$.
The inequality sign, which is placed on the border line between
r5c7 (row 5 column 7) and r5c8, indicates that the number in r5c7
is smaller than the number in r5c8. As this variant has only one
inequality sign on the border line, it is called Minimal Difference
A word document containing the problem can be found here
, for use
in the classroom.