Platon Sudoku · Sudoku Study

Jellyfish · Step-by-step example

For one digit, all candidates in four base rows lie in four cover columns. Rows and columns may be exchanged. Remove that digit from the cover lines outside the four bases.

This course is adapted from Platon Sudoku App. Read the complete example here, then continue learning and practising in the app.

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Recognize the pattern

For one digit, all candidates in four base rows lie in four cover columns. Rows and columns may be exchanged.

How to find it

Scan a single digit across four rows or columns; count every candidate in the selected bases and confirm only four cover lines are used.

What it allows

Remove that digit from the cover lines outside the four bases.

Avoid the mix-up

Four drawn lines alone are not a fish. Every candidate in each base must lie on a chosen cover line.

Step-by-step example · Row

This example starts from a saved intermediate position. Its candidates include earlier logical eliminations; they are not fresh automatic notes.

Blue: evidence. Green: conclusion. Crossed-out digits: eliminated candidates. R1C1 means row 1, column 1.

Step 1 · Recognize the Jellyfish

Each of the four base regions (row 2, row 3, row 6, row 7) still needs 2, and all its circled candidates lie in the same four cover regions (column 3, column 4, column 7, column 8). The “Base lines” legend marks the four base regions, and “Cover lines” marks the four cover regions. Together they form the Jellyfish.

Step 2 · Each cover must be occupied once

The four bases must place four 2 in total. A cover cannot contain the digit twice, and all four placements are confined to these four covers. Therefore the placements occupy every cover exactly once.

Step 3 · Remove candidates outside the bases

Every cover already receives its 2 from one of the four base regions. Therefore candidates in those covers but outside the bases cannot be 2; remove R4C4=2, R9C7=2, R9C8=2.

What it allows

Every cover already receives its 2 from one of the four base regions. Therefore candidates in those covers but outside the bases cannot be 2; remove R4C4=2, R9C7=2, R9C8=2.

Jellyfish · Remove that digit from the cover lines outside the four bases.

Step-by-step example · Column

This example starts from a saved intermediate position. Its candidates include earlier logical eliminations; they are not fresh automatic notes.

Blue: evidence. Green: conclusion. Crossed-out digits: eliminated candidates. R1C1 means row 1, column 1.

Step 1 · Recognize the Jellyfish

Each of the four base regions (column 3, column 4, column 7, column 8) still needs 9, and all its circled candidates lie in the same four cover regions (row 3, row 4, row 7, row 8). The “Base lines” legend marks the four base regions, and “Cover lines” marks the four cover regions. Together they form the Jellyfish.

Step 2 · Each cover must be occupied once

The four bases must place four 9 in total. A cover cannot contain the digit twice, and all four placements are confined to these four covers. Therefore the placements occupy every cover exactly once.

Step 3 · Remove candidates outside the bases

Every cover already receives its 9 from one of the four base regions. Therefore candidates in those covers but outside the bases cannot be 9; remove R4C2=9, R4C6=9, R7C1=9, R7C2=9, R7C6=9, R8C1=9, R8C2=9.