Forcing chain strategy for solving sudoku
Advanced strategies
Forcing chain (XY-chain, Y-cycle)
This technique used same principle than "Excluding chain" strategie. The difference is that here we will not form a chain with the same candidate, but will use diferent candidates to close the chain.
Example
The chain must start from a cell with only two candidates, here we will follow the box in yellow (last column of the first line).
While admitting that the "2" is the good candidate in this box, we follow cells with two candidates, and only two candidates, and we conclude that solution of the green cell is a "6".
Let us take again this same chain since the beginning but by imagining this time that this is the "3" in the yellow cell. While again following the chain, which passes by other cells, we arrive at the same conclusion in the green cell, still one "6".
We know there is one "2" or one "3" in the yellow cell, but whatever the possibility right, in all cases, there is one "6" in the green box, we must thus eliminate the candidate "8" and write a "6".
Strategies list
Elimination of candidates by interaction in the same area
Elimination of candidates by interaction in several areas
Elimination of candidates by interaction in all the puzzle
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