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Single Possibility in Cage

This technique is specific to Killer Sudoku. If we know which values can go in a cage, it is often possible to determine a single valid permutation (a single way to order the values without contradiction) for the whole cage. An example of this is shown below:

We can fill the highlighted cage with a "9" and an "8". The cage has a sum of 17, meaning that it must contain an "8" and a "9" (because this is the only way we can get a sum of 17 from two digits). One of the cells in the cage, however, shares a column with the "9" in the centre of the puzzle. So this cell cannot be a "9", and must be an "8". Similarly, the other cell cannot be an "8" (because the cell next to it must be one) and therefore must be a "9".

This technique is only useful in conjunction with Combination Culling, which as we will see later, allows us to eliminate the digits which can't possibly go in a cage.

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