Proven Sudoku
Techniques / Almost locked sets
Subset Exclusion
Also called General exclusion, Subset counting.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
The same idea for any size of group. Try every combination the group could hold, throw away the impossible ones, and keep what is left.
The exact rule, for stronger players
The rule, stated exactly: For a set of cells, enumerate the candidate assignments consistent with every constraint the set touches. Any candidate present in no consistent assignment is eliminated.
What it removes: Remove candidates absent from every consistent assignment.
Why it works: Exhaustive over the set, sound by construction.
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
- Aligned Pair ExclusionLook at two squares together and list every pair of digits they could hold.
Related almost-locked sets
- Sue de CoqLook where a row or column crosses a box.
- Sue de Coq (extended)The same counting trick with more squares involved.
- Aligned Triple ExclusionLook at three squares together and list every set of three numbers they could hold.
- Almost Hidden SetThe mirror image of an almost locked set.
- Almost Locked CandidatesTwo areas cross.
How often does it come up?
Our rough estimate: the umbrella form. In practice it shades into brute force as the set grows.