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The first square in the top left corner of this sample (1,1) contains a set of two small digits {3 and 8} and the square (1,3) contains a set of three small digits {2, 3 and 9}.
If we colour the squares adjacent to (1,1) with blue, and the squares adjacent to (1,3) with yellow, we find that the square (1,2) is coloured both with blue and yellow. It is the intersection of two sets of coloured squares, hence the name "Intersection Sudoku".
As both squares (1,1) and (1,3) contain the small digit 3 , the number in square (1,2) must be 3. This leaves the square (2,1) with the number 8.