sudokUI › Sudoku techniques › Cell Forcing Chain
Cell Forcing Chain in sudoku
Every candidate of one cell is assumed true in turn, each time placing the naked and hidden singles that follow, and the results are compared.
Why it works
The cell must hold one of its candidates, so one of the runs follows the true case. A digit every run places in the same cell is placed, and a candidate every run removes is eliminated.
How to spot it
Choose a cell with two or three candidates that sees bivalue cells sharing those candidates, so each assumption sets off a run of singles.
The solver uses this on the hardest puzzles. It has no pattern to spot, so it is not offered in practice.
In a puzzle's rating
Each time the solver needs Cell Forcing Chain, the puzzle's difficulty rating grows by 550. Its class is Extreme. It is needed in fewer than 1 in 5,500,000 puzzles that sudokUI generates. How the rating works.
More last resort
Exocet · Double Exocet · Pattern Overlay · Forcing Chain · Digit Forcing Chain · Nishio Forcing Chain · Unit Forcing Chain · Forcing Net · Brute Force