Proven Sudoku
Reverse BUG
Also called Reverse Bivalue Universal Grave, Reverse BUG pattern.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
This one uses the numbers you have already filled in, not the ones still open. If a set of them could be swapped to make a second answer, that cannot be right. So the swap gets ruled out.
The exact rule, for stronger players
The rule, stated exactly: The BUG argument applied to placed digits rather than candidates, in the manner of an Avoidable Rectangle. A configuration of solved non-given cells that could permute is impossible.
What it removes: Remove candidates that would produce the permutable configuration.
Why it works: Solved non-given cells are deductions. A permutable set of them implies multiple solutions. 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.
- Avoidable RectangleFour squares can form a rectangle across two boxes.
Related uniqueness
- Unique LoopLike a rectangle but longer, a ring of squares all sharing two numbers.
- Unique RectangleFour squares in a rectangle across two boxes, all holding the same two numbers.
- Unique Rectangle Type 1Three corners of the rectangle hold only the two numbers.
- Unique Rectangle Type 2Two corners hold only the pair.
- Unique Rectangle Type 3Two corners hold the pair.
How often does it come up?
Our rough estimate: rare, and depends on which cells were givens.