Proven Sudoku
BUG+1
Also called BUG +1, Bivalue Universal Grave plus one.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
Say every open square had two numbers left, and each number had two homes in every group. That grid would have two answers. One square breaks the pattern. The number that breaks it must be the right one.
The exact rule, for stronger players
The rule, stated exactly: Every unsolved cell has exactly two candidates except one, which has three. The digit appearing three times in a house through that cell must be placed there.
What it removes: Place the digit that appears an odd number of times in the trivalue cell's houses.
Why it works: Removing the extra candidate would leave a BUG, which cannot occur. Therefore the extra candidate is the solution of that cell. This technique is only valid when the puzzle is known to have exactly one solution. It is not a deduction about Sudoku rules, it is a deduction about the puzzle setter's promise. On a grid with multiple solutions it produces wrong answers.
Worked examples
None yet. This pattern does not turn up in our daily boards, so there is no real example to show. We would rather wait than draw a fake one.
Learn these first
- Bivalue Universal GraveSay every open square had just two numbers left.
Related uniqueness
- BUG+2The same idea when two squares break the pattern instead of one.
- BUG+3The same again with three odd squares.
- Deadly PatternA shape that would give the puzzle two answers if it ever appeared.
- Extended Unique RectangleA bigger version using six squares and three numbers instead of four and two.
- Hidden Unique RectangleThe rectangle trick again, but the giveaway is hidden.
How often does it come up?
Our rough estimate: uncommon but decisive when it appears, and very fast to apply once the grid is nearly all bivalue.