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'Rectangle Outline Sudoku' printed from http://nrich.maths.org/
Invented by Henry Kwok
Rules of Rectangle Outline Sudoku
This Sudoku variant has two additional rules.
As usual, the object of the puzzle is to fill in the whole 9x9 grid
with numbers 1 through 9 so that each row, each column and each box
must contain all the nine different numbers 1 to 9 irrespective of
order.
The two additional rules of the variant are as follows:

Each of the two main diagonals of the puzzle must have the
numbers 1 to 9.

Within the 9x9 pattern there is a highlighted "rectangle" with
its width four and length six cells respectively. Each width of the
"rectangle" outline must contain all the four numbers 1 to 4.
Similarly, each length of the "rectangle" must contain all the six
numbers 1 to 6.