Proven Sudoku
Kraken
Also called Kraken Cell, Kraken chain.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
Take a pattern that nearly works. Then check each way it could go wrong. If every one of those ways ends in the same place, you can trust that place anyway.
The exact rule, for stronger players
The rule, stated exactly: A case split in which every possible truth assignment for some structure, a fish fin set or a cell's candidate list, leads by chain to the same elimination. The elimination is then unconditional.
What it removes: Remove the candidate that every branch excludes.
Why it works: If every case implies the same conclusion, the conclusion holds regardless of which case is true. This is proof by exhaustive cases rather than by a single chain.
Worked examples
None yet. This pattern does not turn up in our daily boards, so there is no real example to show. We would rather wait than draw a fake one.
Learn these first
- Grouped AICA chain where a step can use a whole group of squares at once instead of a single square.
- Complex Fish (rank framework)The fish idea, but boxes are allowed to join the rows and columns.
Related chains
- NishioAssume a number goes in a square, then check whether the rest of that number can still fit everywhere it needs to.
- Nishio Forcing NetThe same check as Nishio.
- Unit Forcing ChainTake one number in one row, column or box and try each home it could have.
- X-ChainA chain about one number only.
- XY-ChainA chain of squares that each hold exactly two numbers.
How often does it come up?
Our rough estimate: uncommon, and often the last resort before brute force.