Proven Sudoku
Techniques / Exotic and rank-zero
SK Loop
Also called Stephen Kurzhals Loop, Domino Loop (related).
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
A ring of almost complete sets passing through the corners of a band and a stack. Each set feeds the next around the ring. It clears a lot at once, and it is hard to spot.
The exact rule, for stronger players
The rule, stated exactly: A closed loop of eight intersection regions around the four corner boxes, where each region's digits pair with the next, producing a rank-zero structure with extensive eliminations.
What it removes: Remove the loop digits from cells of the involved houses outside the loop regions.
Why it works: The loop forces each intersection to supply a fixed count of digits to its neighbours. Rank-zero counting then saturates the covers.
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
- Rank TheoryA way to count a pattern.
Related exotic and rank-zero
- Table-Driven MethodsWork out ahead of time what follows from each single guess, and store it in a table.
- Broken WingA ring of squares where one number would have nowhere to go if the ring were perfect.
- Domino LoopA ring of pairs that lean on each other all the way round, like falling dominoes.
- Double ExocetTwo of those launcher patterns sharing the same numbers.
- FireworksA number is trapped so that it can only sit in one row, one column, or the square where they cross.
How often does it come up?
Our rough estimate: rare, and largely confined to specific hard puzzle families.