Sudoku Solver

Extreme

Jellyfish (Column)

A 4x4 pattern where a candidate appears in only four columns, restricted to the same four rows.

Jellyfish (Column) is an extreme strategy that expands on the logic of the Swordfish (Column) and X-Wing (Column). It uses a 4x4 pattern involving four columns and four rows.

It proves that "since the numbers for these 4 columns MUST be in these 4 rows, no other cells in those rows can have that number."

Interactive Example

1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
5
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
4
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
5
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
3
1 2 3 4 5 6 7 8 9
2
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
2
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
3
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
5
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
7
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
5
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
7
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1
1 2 3 4 5 6 7 8 9
9
1 2 3 4 5 6 7 8 9
2
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
6
1 2 3 4 5 6 7 8 9

Click "Apply Logic" to see the strategy in action.

Real Example Walkthrough

In the example puzzle above, the strategy targets the number 8:

1. The Base Sets (Columns) We look at four specific columns: - Column 2 - Column 6 - Column 7 - Column 8

In these four columns, the candidate 8 appears only in four rows: - Row 3 - Row 5 - Row 8 - Row 9

2. The Pattern Let's map out where 8 can go in our base columns: - Col 2: Restricted to Rows 3, 5, 8, 9 - Col 6: Restricted to Rows 3, 5, 8, 9 - Col 7: Restricted to Rows 3, 5, 8, 9 - Col 8: Restricted to Rows 3, 5, 8, 9

3. The Logic - We need to place four 8s—one for Column 2, Column 6, Column 7, and Column 8. - We have exactly four rows available for them (Rows 3, 5, 8, 9). - Therefore, these four rows must contain the 8s for our four base columns. - Consequently, no other cell in Row 3, Row 5, Row 8, or Row 9 can contain an 8.

4. The Elimination We can eliminate 8 from any cell in the cover rows (3, 5, 8, 9) that is NOT part of our Jellyfish pattern. - Eliminate 8 from R9C5 inside Row 9.

How to Spot a Jellyfish

Jellyfish are incredibly rare and difficult to spot.

  1. Systematic Scanning: You almost certainly need candidate highlighting.
  2. Count Candidates: Look for columns with only 2, 3, or 4 of a specific candidate.
  3. Find the Fit: You need 4 columns whose candidates collectively fit into exactly 4 rows.
    • Not every column needs a candidate in every row. As long as the union of rows occupied by the 4 columns is exactly 4 rows, it works.

Visual Guide

C1 C2 C3 C4 R1 X X . . <- Row 1 Cover R2 . X X . <- Row 2 Cover R3 . . X X <- Row 3 Cover R4 X . . X <- Row 4 Cover ^ ^ ^ ^ Base Cols

  • 4 Base Columns restrict candidates to 4 Cover Rows.
  • Eliminate candidates from the Cover Rows horizontally.

Common Mistakes

  • Seeing Patterns That Aren't There: It's easy to miss a candidate hiding in a 5th row, which invalidates the pattern.
  • Eliminating Vertically: Remember, if you start with Columns, you eliminate Horizontally (Rows).

Comparison Table

Strategy Pattern Size Base Sets Cover Sets Frequency
X-Wing 2x2 2 Columns 2 Rows Common
Swordfish 3x3 3 Columns 3 Rows Rare
Jellyfish 4x4 4 Columns 4 Rows Very Rare

Related Strategies