sudokUI › Sudoku techniques › Forcing Chain
Forcing Chain in sudoku
One assumption, or each of a set of assumptions that covers every case, such as all candidates of one cell, is followed through the steps it forces.
Why it works
One of a set of assumptions covering every case must be true, so any placement or elimination that every one of them forces is certain. An assumption that leads to a contradiction is false: a candidate assumed true is eliminated, one assumed false is placed.
How to spot it
Begin where the choice is smallest, a bivalue cell or a conjugate pair, and pencil each run in its own colour to compare them.
Not implemented in sudokUI: the chain engines already find everything it can.
In a puzzle's rating
Each time the solver needs Forcing Chain, the puzzle's difficulty rating grows by 500. Its class is Extreme. How the rating works.
More last resort
Exocet · Double Exocet · Pattern Overlay · Digit Forcing Chain · Nishio Forcing Chain · Cell Forcing Chain · Unit Forcing Chain · Forcing Net · Brute Force