sudokUI › Sudoku techniques › Unit Forcing Chain
Unit Forcing Chain in sudoku
Every place for a digit X in one unit is assumed true in turn, each time placing the naked and hidden singles that follow, and the results are compared.
Why it works
X must go somewhere in the unit, 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
Start with a conjugate pair, which needs only two runs, then try digits with three places left in a unit.
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 Unit 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 · Cell Forcing Chain · Forcing Net · Brute Force