Jellyfish

Level 11 · Red Belt

Strategy:

Requires:

Learn after:

Finned Jellyfish Squirmbag

Overview

Jellyfish is the four-row, four-column extension of fish patterns. When a candidate in four rows is restricted to the same four columns, it can be eliminated from other cells in those columns.

How It Works

Find four rows where a candidate appears, with all occurrences in the same four columns.
Each row must have 2-4 occurrences of the candidate within these columns.
Eliminate the candidate from other cells in those four columns.

Example

Starting position

Look for Jellyfish.

Sudoku grid showing the initial puzzle state for Look for Jellyfish.

1

Scan base columns 6, 7, 8 and 9 for 7

Base columns: 6, 7, 8 and 9. Check where digit 7 can appear in those columns.

Sudoku grid illustrating step 1: Scan base columns 6, 7, 8 and 9 for 7 — Base columns: 6, 7, 8 and 9. Check where digit 7 can appear in those columns.

2

r2c6 is already filled

r2c6 is already filled with 9.

Sudoku grid illustrating step 2: r2c6 is already filled — r2c6 is already filled with 9.

3

7 is blocked from r3c6 by the row

Digit 7 cannot go in r3c6 because r3c2 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 3: 7 is blocked from r3c6 by the row — Digit 7 cannot go in r3c6 because r3c2 is already in this row. Remove the entry or choose a different digit.

4

r4c6 is already filled

r4c6 is already filled with 3.

Sudoku grid illustrating step 4: r4c6 is already filled — r4c6 is already filled with 3.

5

7 is blocked from r7c6 by row and box

Digit 7 cannot go in r7c6 because r7c1 is in this row and r8c4 is in this box. Remove the entry or choose a different digit.

Sudoku grid illustrating step 5: 7 is blocked from r7c6 by row and box — Digit 7 cannot go in r7c6 because r7c1 is in this row and r8c4 is in this box. Remove the entry or choose a different digit.

6

7 is blocked from r8c6 by the row

Digit 7 cannot go in r8c6 because r8c4 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 6: 7 is blocked from r8c6 by the row — Digit 7 cannot go in r8c6 because r8c4 is already in this row. Remove the entry or choose a different digit.

7

7 was already removed from r2c7

Digit 7 is not a current candidate in r2c7 because earlier solving steps already removed it.

Sudoku grid illustrating step 7: 7 was already removed from r2c7 — Digit 7 is not a current candidate in r2c7 because earlier solving steps already removed it.

8

7 is blocked from r3c7 by the row

Digit 7 cannot go in r3c7 because r3c2 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 8: 7 is blocked from r3c7 by the row — Digit 7 cannot go in r3c7 because r3c2 is already in this row. Remove the entry or choose a different digit.

9

r4c7 is already filled

r4c7 is already filled with 5.

Sudoku grid illustrating step 9: r4c7 is already filled — r4c7 is already filled with 5.

10

r7c7 is already filled

r7c7 is already filled with 1.

Sudoku grid illustrating step 10: r7c7 is already filled — r7c7 is already filled with 1.

11

r8c7 is already filled

r8c7 is already filled with 6.

Sudoku grid illustrating step 11: r8c7 is already filled — r8c7 is already filled with 6.

12

7 was already removed from r2c8

Digit 7 is not a current candidate in r2c8 because earlier solving steps already removed it.

Sudoku grid illustrating step 12: 7 was already removed from r2c8 — Digit 7 is not a current candidate in r2c8 because earlier solving steps already removed it.

13

r3c8 is already filled

r3c8 is already filled with 1.

Sudoku grid illustrating step 13: r3c8 is already filled — r3c8 is already filled with 1.

14

7 is blocked from r4c8 by the row

Digit 7 cannot go in r4c8 because r4c3 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 14: 7 is blocked from r4c8 by the row — Digit 7 cannot go in r4c8 because r4c3 is already in this row. Remove the entry or choose a different digit.

15

r7c8 is already filled

r7c8 is already filled with 2.

Sudoku grid illustrating step 15: r7c8 is already filled — r7c8 is already filled with 2.

16

7 is blocked from r8c8 by the row

Digit 7 cannot go in r8c8 because r8c4 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 16: 7 is blocked from r8c8 by the row — Digit 7 cannot go in r8c8 because r8c4 is already in this row. Remove the entry or choose a different digit.

17

7 was already removed from r2c9

Digit 7 is not a current candidate in r2c9 because earlier solving steps already removed it.

Sudoku grid illustrating step 17: 7 was already removed from r2c9 — Digit 7 is not a current candidate in r2c9 because earlier solving steps already removed it.

18

r3c9 is already filled

r3c9 is already filled with 2.

Sudoku grid illustrating step 18: r3c9 is already filled — r3c9 is already filled with 2.

19

7 is blocked from r4c9 by the row

Digit 7 cannot go in r4c9 because r4c3 is already in this row. Remove the entry or choose a different digit.

Sudoku grid illustrating step 19: 7 is blocked from r4c9 by the row — Digit 7 cannot go in r4c9 because r4c3 is already in this row. Remove the entry or choose a different digit.

20

r7c9 is already filled

r7c9 is already filled with 8.

Sudoku grid illustrating step 20: r7c9 is already filled — r7c9 is already filled with 8.

21

r8c9 is already filled

r8c9 is already filled with 3.

Sudoku grid illustrating step 21: r8c9 is already filled — r8c9 is already filled with 3.

22

7 is confined to rows 1, 5, 6 and 9

In each of these 4 columns, digit 7 is confined to the same 4 cover rows: 1, 5, 6 and 9.

Sudoku grid illustrating step 22: 7 is confined to rows 1, 5, 6 and 9 — In each of these 4 columns, digit 7 is confined to the same 4 cover rows: 1, 5, 6 and 9.

23

The pattern forms a Jellyfish

That forms a Jellyfish: the cover rows must hold the digit for the base columns, so other columns cannot keep it there.

Sudoku grid illustrating step 23: The pattern forms a Jellyfish — That forms a Jellyfish: the cover rows must hold the digit for the base columns, so other columns cannot keep it there.

24

Extra 7 candidates in rows 1, 5, 6 and 9

These are the extra 7 candidates in rows 1, 5, 6 and 9 that must be removed: r6c5.

Sudoku grid illustrating step 24: Extra 7 candidates in rows 1, 5, 6 and 9 — These are the extra 7 candidates in rows 1, 5, 6 and 9 that must be removed: r6c5.

25

Remove 7 from rows 1, 5, 6 and 9

Remove 7 from the extra cells in rows 1, 5, 6 and 9: r6c5.

Sudoku grid illustrating step 25: Remove 7 from rows 1, 5, 6 and 9 — Remove 7 from the extra cells in rows 1, 5, 6 and 9: r6c5.

26

Result

Result after applying the technique.

Sudoku grid showing the result after applying the technique

Tips & Patterns

Jellyfish are quite rare - master X-Wing and Swordfish first.
Computer assistance is helpful for finding Jellyfish reliably.
The pattern may not use all 16 intersection cells.