Finned Squirmbag

Level 11 · Red Belt

Strategy:

Finned & Sashimi Fish

Requires:

Finned Jellyfish Squirmbag

Overview

A Finned Squirmbag is a Squirmbag (5-fish) pattern with extra candidates forming a fin. This is one of the rarest patterns in Sudoku solving.

How It Works

Find a Squirmbag where one row has candidates outside the five columns.
These extra cells are the fin.
Eliminations occur only in cells seeing both the cover columns and the fin.

Example

Look for Finned Squirmbag.

Sudoku grid showing the initial puzzle state for Look for Finned Squirmbag.

1

Finned Squirmbag

Check digit 8 in rows 1, 5, 6, 8 and 9. These rows form a Squirmbag pattern in columns 2, 6, 7, 8 and 9.

Sudoku grid illustrating step 1: Finned Squirmbag — Check digit 8 in rows 1, 5, 6, 8 and 9. These rows form a Squirmbag pattern in columns 2, 6, 7, 8 and 9.

2

Finned Squirmbag

Digit 8 is not a current candidate in r1c1 because earlier solving steps already removed it.

Sudoku grid illustrating step 2: Finned Squirmbag — Digit 8 is not a current candidate in r1c1 because earlier solving steps already removed it.

3

Finned Squirmbag

Digit 8 is not a current candidate in r1c3 because earlier solving steps already removed it.

Sudoku grid illustrating step 3: Finned Squirmbag — Digit 8 is not a current candidate in r1c3 because earlier solving steps already removed it.

4

Finned Squirmbag

r1c4 is already filled with 1.

Sudoku grid illustrating step 4: Finned Squirmbag — r1c4 is already filled with 1.

5

Finned Squirmbag

r1c5 is already filled with 4.

Sudoku grid illustrating step 5: Finned Squirmbag — r1c5 is already filled with 4.

6

Finned Squirmbag

r5c4 is already filled with 4.

Sudoku grid illustrating step 6: Finned Squirmbag — r5c4 is already filled with 4.

7

Finned Squirmbag

Digit 8 cannot go in r5c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 7: Finned Squirmbag — Digit 8 cannot go in r5c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

8

Finned Squirmbag

r6c1 is already filled with 4.

Sudoku grid illustrating step 8: Finned Squirmbag — r6c1 is already filled with 4.

9

Finned Squirmbag

r6c3 is already filled with 6.

Sudoku grid illustrating step 9: Finned Squirmbag — r6c3 is already filled with 6.

10

Finned Squirmbag

r6c4 is already filled with 2.

Sudoku grid illustrating step 10: Finned Squirmbag — r6c4 is already filled with 2.

11

Finned Squirmbag

Digit 8 cannot go in r6c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 11: Finned Squirmbag — Digit 8 cannot go in r6c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

12

Finned Squirmbag

r8c1 is already filled with 6.

Sudoku grid illustrating step 12: Finned Squirmbag — r8c1 is already filled with 6.

13

Finned Squirmbag

r8c3 is already filled with 2.

Sudoku grid illustrating step 13: Finned Squirmbag — r8c3 is already filled with 2.

14

Finned Squirmbag

r8c4 is already filled with 5.

Sudoku grid illustrating step 14: Finned Squirmbag — r8c4 is already filled with 5.

15

Finned Squirmbag

Digit 8 cannot go in r8c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

Sudoku grid illustrating step 15: Finned Squirmbag — Digit 8 cannot go in r8c5 because r2c5 is already in this column. Remove the entry or choose a different digit.

16

Finned Squirmbag

r9c1 is already filled with 3.

Sudoku grid illustrating step 16: Finned Squirmbag — r9c1 is already filled with 3.

17

Finned Squirmbag

r9c3 is already filled with 5.

Sudoku grid illustrating step 17: Finned Squirmbag — r9c3 is already filled with 5.

18

Finned Squirmbag

r9c4 is already filled with 6.

Sudoku grid illustrating step 18: Finned Squirmbag — r9c4 is already filled with 6.

19

Finned Squirmbag

r9c5 is already filled with 9.

Sudoku grid illustrating step 19: Finned Squirmbag — r9c5 is already filled with 9.

20

Finned Squirmbag

One base row has extra candidates in box 4. That box is the fin, so this becomes a Finned Squirmbag.

Sudoku grid illustrating step 20: Finned Squirmbag — One base row has extra candidates in box 4. That box is the fin, so this becomes a Finned Squirmbag.

21

Finned Squirmbag

Because of the fin, only cells in box 4 that also lie in columns 2, 6, 7, 8 and 9 can lose digit 8.

Sudoku grid illustrating step 21: Finned Squirmbag — Because of the fin, only cells in box 4 that also lie in columns 2, 6, 7, 8 and 9 can lose digit 8.

22

Finned Squirmbag

These are the cells that see both the fish structure and the fin: r4c2.

Sudoku grid illustrating step 22: Finned Squirmbag — These are the cells that see both the fish structure and the fin: r4c2.

23

Finned Squirmbag

Remove 8 from: r4c2.

Sudoku grid illustrating step 23: Finned Squirmbag — Remove 8 from: r4c2.

24

Result after applying the technique.

Sudoku grid showing the result after applying the technique

Tips & Patterns

Finned Squirmbags are almost never needed in human-solvable puzzles.
This is primarily of theoretical interest.