Contradiction
Suppose the cell were rejected (or kept) and follow the consequences — if the rules break, the opposite must be true.
How it works
- 1Nothing direct is left here. The 5 between the two 4s was settled by the sandwich rule, and no technique above this one reaches any of the other eight cells.
- 2So suppose an answer and follow it: imagine the highlighted 3 is rejected, then apply the ordinary techniques from there and see how far they run.
- 3If the supposition ends up breaking a rule — two rejected cells touching, a duplicate kept, or the board cut in two — it was wrong, and the cell is the opposite of what you supposed.
- 4This is the last resort. Try every direct technique first, and only then start supposing.
- Kept
- Cells that establish the pattern