BUG+1
Bivalue Universal Grave + 1
Every empty cell has two candidates except one with three: that cell takes the digit appearing three times in its units.
Rule
All cells have 2 candidates, one cell has 3 → it takes the digit that appears 3 times in its row, column and box.
Almost all bivalue cells
Every empty cell has exactly two candidates, except r8c8, which has three: 3, 6 and 8.
The idea
A BUG is a state where every empty cell has exactly two candidates and every digit appears exactly twice in each unit. Such a state never has exactly one solution.
When the board is just one candidate away from a BUG — one cell with three, all others with two — the extra candidate must be that cell's answer, otherwise you fall into the BUG. The extra candidate is easy to find: it appears three times in the cell's row, column and box.
How to spot it
- It tends to appear at the end of hard puzzles, when the board is almost all bivalue cells.
- Count candidates in the row of the three-candidate cell: the digit that appears three times is the answer.
Common mistakes
- It applies only when exactly one cell has three candidates and every other empty cell has exactly two.
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Related techniques
References
The examples on this page are generated from random puzzles, not taken from these sources.