sudokUI › Sudoku techniques › Forcing Net
Forcing Net in sudoku
One candidate is assumed true and followed through locked candidates as well as naked and hidden singles, any step drawing on several earlier ones, until a contradiction appears.
Why it works
Every step is a certain consequence of the assumption, so a contradiction, an empty cell without candidates or a unit with no place for a digit, proves the assumption false. Only the assumed candidate is eliminated.
How to spot it
When singles stall without a contradiction, look for a digit confined in some unit to the cells where a box crosses a row or column.
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 Forcing Net, the puzzle's difficulty rating grows by 700. 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 · Unit Forcing Chain · Brute Force