Swordfish (Sudoku)
The method swordfish is a technical of resolution of Sudoku. It allows the elimination of several candidates and is in the grids of level of high difficulty. This model is a variation of the X-wing.
Principle
For a specific candidate, a swordfish is consisted three lines, each one not containing more than three boxes with the candidate, and which divide the three same columns.The model, thus determined, thus represents a grid of 9 boxes of which some can not contain the candidate.
All the candidates being on the columns and external with the model can be excluded without risk.
The model can obviously function in the other direction, by reversing lines and columns.
This approach can be generalized for grids of 16 and 25 boxes but it is not sure that the case exists.
Illustration
- In this example the coloured boxes are those comprising candidate 2, except the 8 and the 9, put in blue for a better visibility.
Trois lines (3, 5 and 7) has each one only 2 or 3 boxes with candidate 2; These candidates are in the same columns.
- Swordfish is formed, it is represented by the blue boxes.
- the 5 candidates not forming part of the swordfish and located in the same columns (3, 7 and 9), ringed yellows, can be eliminated without risk.
See too
Technique of the X-wing