Nishio Forcing
Suppose one candidate and propagate only forced singles; if the chain reaches an impossible cell or duplicate, that candidate is ruled out.
How it works
- 1A1 has two candidates, 4 and 7, and no other technique can decide between them.
- 2Suppose 4 and push the easy consequences through. If that ends with some cell holding zero legal candidates — like D1 here — then 4 was impossible.
- 3So strike 4 from A1 and carry on. This is a real deduction, not a guess you keep: the refuted digit is gone for good, and only the surviving candidate had to be true. It stays the last resort — every technique above exists to avoid needing it.
- Second colour in the chain
- Where the conclusion lands
- Candidate ruled out