Swordfish strategy for solving sudoku
Advanced strategies
Swordfish (3 arms star formation)
Swordfish is the same thing as the X-wing but this time we will work not with only one square but with two sqares overlapped.
It is thus necessary to find 3 lines or 3 columns which contain each one twice the same candidate, and only two, and that these candidates are aligned with the others two to two.
Under this condition, all the candidates being in the prolongation of the columns or the lines respectively are to be eliminated.
Example
This grid presents a Swordfish.
Indeed, three columns (2, 5 and 8) contain each one only twice the candidate "3". The candidates "3" of column 2 are aligned with one of column 5 and one of the column 8 and one of the candidates of column 5 is aligned with column 8.
If we adopt an approach, we find the three positions for the candidate "3".
And by adopting the opposed approach, we find the three other positions.
Whatever these positions, all the other candidates being on the lines of our swordfish are to be eliminated.
Strategies list
Elimination of candidates by interaction in the same area
Elimination of candidates by interaction in several areas
Elimination of candidates by interaction in all the puzzle
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