sudokUI › Sudoku techniques › Jellyfish
Jellyfish in sudoku
4-fish · Naked Quadruple seen sideways · how it fits
Digit X has two to four candidates in each of four rows, all lying in the same four columns, or likewise with rows and columns swapped.
Why it works
Each of the four rows places X in a different one of the four columns, so all four columns get their X from these rows. Remove X from all other cells in those columns, or in those rows for the swapped form.
How to spot it
Filter on one digit and find four rows whose candidates together span only four columns: a row may use just two of them.
Worked example
Jellyfish on 6: in rows 2, 6, 8, 9, 6 can only go in columns 5, 6, 8, 9 (r2c5, r2c9, r6c5, r6c8, r8c6, r8c9, r9c6, r9c8). Those 4 rows will use up 6 in those 4 columns, so 6 can go nowhere else in them. So 6 is removed from r3c9.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Jellyfish and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs Jellyfish, the puzzle's difficulty rating grows by 160. Its class is Hard. It is needed in 1 in 1,500 puzzles that sudokUI generates. How the rating works.