Finned Squirmbag

Level 12 · Black 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

Starting position

Look for Finned Squirmbag.

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

1

Rows 1, 5, 6, 8 and 9 form a Squirmbag on 8

Base rows 1, 5, 6, 8 and 9 form a Squirmbag for digit 8 in cover columns 2, 6, 7, 8 and 9.

Sudoku grid illustrating step 1: Rows 1, 5, 6, 8 and 9 form a Squirmbag on 8 — Base rows 1, 5, 6, 8 and 9 form a Squirmbag for digit 8 in cover columns 2, 6, 7, 8 and 9.

2

8 was already removed from r1c1

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

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

3

8 was already removed from r1c3

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

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

4

r1c4 is already filled

r1c4 is already filled with 1.

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

5

r1c5 is already filled

r1c5 is already filled with 4.

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

6

r5c4 is already filled

r5c4 is already filled with 4.

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

7

8 is blocked from r5c5 by the column

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

8

r6c1 is already filled

r6c1 is already filled with 4.

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

9

r6c3 is already filled

r6c3 is already filled with 6.

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

10

r6c4 is already filled

r6c4 is already filled with 2.

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

11

8 is blocked from r6c5 by the column

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

12

r8c1 is already filled

r8c1 is already filled with 6.

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

13

r8c3 is already filled

r8c3 is already filled with 2.

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

14

r8c4 is already filled

r8c4 is already filled with 5.

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

15

8 is blocked from r8c5 by the column

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

16

r9c1 is already filled

r9c1 is already filled with 3.

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

17

r9c3 is already filled

r9c3 is already filled with 5.

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

18

r9c4 is already filled

r9c4 is already filled with 6.

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

19

r9c5 is already filled

r9c5 is already filled with 9.

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

20

The fin sits in box 4

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

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

21

Only box 4 can lose 8

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: Only box 4 can lose 8 — 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

Cells that see both fish and fin

These cells see both the fish structure and the fin, so they are removal targets: r4c2.

Sudoku grid illustrating step 22: Cells that see both fish and fin — These cells see both the fish structure and the fin, so they are removal targets: r4c2.

23

Remove 8 from the fin-affected cells

Remove 8 from these fin-affected cells: r4c2.

Sudoku grid illustrating step 23: Remove 8 from the fin-affected cells — Remove 8 from these fin-affected cells: r4c2.

24

Result

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.