The Finned Swordfish Technique in Sudoku
What Is a Finned Swordfish?
A Finned Swordfish is a three-by-three Swordfish with one or more extra positions for the same candidate in one base unit. The extra candidate is the fin. If the fin is false, the regular Swordfish remains; if it is true, it directly controls cells in its block. Eliminations are therefore limited to cells affected in both branches.
How to Identify a Finned Swordfish
- Choose one candidate digit.
- Find three base rows whose main positions fit within three cover columns.
- Locate an extra candidate outside the cover columns in one base row.
- Confirm that the fin shares a block with a body candidate on a cover column.
- Eliminate from that cover column inside the fin block.
Candidate 9 forms the Swordfish body in rows 2, 5, and 8 and columns 2, 5, and 8. R5C9 is the fin beside body cell R5C8. Whether the fin is true or the regular Swordfish remains, candidate 9 can be removed from R4C8 and R6C8.
Why the Elimination Is Valid
When the fin is false, the Swordfish occupies all three cover columns and removes other 9s there. When the fin is true, it removes 9 from other cells in its block. The overlap of those two conclusions is the valid elimination area.
Finned Swordfish and Swordfish
A standard Swordfish has all body candidates contained in three cover units. The finned version allows extras but restricts eliminations to the fin block.
What Are Common Finned Swordfish Mistakes?
The fin must belong to the same candidate and base unit as the fish body. Do not apply full-column eliminations; only the overlap between the fish elimination and the fin's block is valid.
Checking Both Logical Branches
If R5C9 is not 9, rows 2, 5, and 8 form a regular Swordfish over columns 2, 5, and 8. If the fin is 9, it controls block 6. R4C8 and R6C8 lie in both the Swordfish elimination area and the fin block, so both branches remove them.
Frequently Asked Questions
Is a Finned Swordfish stronger than Swordfish?
Not necessarily. It tolerates an extra candidate, but its valid elimination area is much smaller.
Can it be column-based?
Yes. Exchange rows and columns while preserving the body, fin, block, and overlap conditions.
Summary
Verify the three-by-three body first, then the fin. Eliminate only where the standard fish conclusion and the fin's block agree.