sudokUI › Sudoku techniques › SK Loop
SK Loop in sudoku
Four solved cells forming a rectangle in four boxes; their rows and columns inside those boxes give eight cell pairs, each holding four candidates, two shared with each neighbouring pair.
Why it works
Neighbouring pairs share a box, row or column, so each shared digit fits only once in their four cells. Eight such links fill at most sixteen cells, exactly the loop's size, so every shared digit is used and eliminated from the rest of that unit.
How to spot it
Look for four solved cells forming a rectangle across four boxes, each with its row and column inside the box otherwise unsolved.
Not implemented in sudokUI: the chain engines already find everything it can.
In a puzzle's rating
Each time the solver needs SK Loop, the puzzle's difficulty rating grows by 400. Its class is Extreme. How the rating works.