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Finned Jellyfish in sudoku

Unfair · rating cost 250 · Finned Fish

Four rows (or columns) have candidate X only in four shared columns (or rows) plus fins, extra cells all in one box, with at least two non-fin candidates per row.

Why it works

If no fin is true, a plain Jellyfish removes X elsewhere in its columns; if one is, its box holds no other X. Cells of those columns in the fin box, outside the four rows, lose X, or likewise with rows and columns swapped.

How to spot it

Find four rows whose candidates for X fit four columns except for a few extras, all in one box crossed by one of those columns.

Worked example

Finned Jellyfish example on a sudoku board. Finned Jellyfish on 1: in rows 1, 4, 7, 8, 1 sits in columns 1, 3, 4, 8 (r1c1, r1c4, r4c1, r4c8, r7c3, r7c4, r8c3, r8c8) except for the fin r4c7. Either the fin is false and the fish holds, or the fin is true; a 1 that sees every fin and lies in the cover columns outside the base is false both ways. So 1 is removed from r5c8.
remove base rows: the 1s of the fish cover columns: where the base's 1s must land fin: 1s outside the fish, in one box; a removal must see every fin

Finned Jellyfish on 1: in rows 1, 4, 7, 8, 1 sits in columns 1, 3, 4, 8 (r1c1, r1c4, r4c1, r4c8, r7c3, r7c4, r8c3, r8c8) except for the fin r4c7. Either the fin is false and the fish holds, or the fin is true; a 1 that sees every fin and lies in the cover columns outside the base is false both ways. So 1 is removed from r5c8.

Step 31 of this puzzle's solution, found and verified by the sudokUI engine. Cells are named by row and column: r2c3 is row 2, column 3. Play this puzzle from the start.

Practise it on a real puzzle. sudokUI generates a puzzle that needs Finned Jellyfish and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.

Practice Finned JellyfishOpen in the app's guide

In a puzzle's rating

Each time the solver needs Finned Jellyfish, the puzzle's difficulty rating grows by 250. Its class is Unfair. It is needed in 1 in 860 puzzles that sudokUI generates. How the rating works.

More finned fish

Finned X-Wing · Sashimi X-Wing · Finned Swordfish · Sashimi Swordfish · Sashimi Jellyfish

← Sashimi SwordfishSashimi Jellyfish →