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'Intersection Sudoku 2' printed from https://nrich.maths.org/
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by Henry Kwok
It has two basic rules:
The Rules of Intersection Sudoku
Like the standard sudoku, this sudoku variant consists of a grid of nine rows and nine columns subdivided into nine 3x3 subgrids.It has two basic rules:
- Each column, each row, and each box (3x3 subgrid) must have the numbers 1 through 9.
- No column, row or box can have two squares with the same number.
The puzzle can be solved with the help of small digits in the
top parts of certain squares.
The small digits stand for the numbers in the squares
horizontally and vertically adjacent to the square that contains
the set of small digits. The number of small digits in each set can
range from a minimum of 2 (in a corner of the puzzle) to a maximum
of 4 (in any square beyond the edge of the puzzle).
A Short Demonstration
The square (3, 6) and the square (5, 6) of this puzzle contain
a set of four small digits {2, 2, 3, 6} and {1, 2, 4, 5}
respectively. If we colour the squares vertically and horizontally
adjacent to (3, 6) with blue, and the squares vertically and
horizontally adjacent to (5, 6) with yellow, we find that the
square (4, 6) is coloured both with blue and yellow. It is the
intersection of two sets of coloured squares, hence the name
"Intersection Sudoku".