Excluding chain strategy for solving sudoku
Advanced strategies
Excluding chain
One of the basic rule of Sudoku is that the same number cannot be found in double in the same area. When we find the same candidate in two cells of an area (and only in two cells), then these candidates form a pair. If one is in the good place, the other must be eliminated and the reverse is true.
It can be possible to connect the pairs and to find a candidate who is false for both starting assumption. This candidate must then be eliminated.
Example
Let us examine all candidatse 8 in this puzzle. We will build our assumptions starting from the first block.
First assumption, we will admit that the candidate in the first column and first line is the good. That makes it possible to then determine the correct values (in green) and incorrect (in red) for other candidates.
Second assumption, we will admit the other candidate of the studied area is the good (the one in second column and second line). That makes it possible to determine the correct and incorrect values for the other candidates.
For both assumptions, the candidate in bottom of the last column is always incorrect.
It should be eliminated.
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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