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`.........`

..A....B.

..B....A.

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Vulnerable cells in a solution pattern set are pairings of cells which have a certain relationship to each other that may be easily exploited.

Vulnerable points in the solution pattern set must fulfill criteria 1 and either criteria 2 or 3, or both.

Criteria 1: The requirement, a pair of vulnerable cells must not lie in the same house; that is, they must not have either a row, column, or box in common. Simpler methods have already reduced things as much as possible for cells in the same house, so you are not likely to learn anything more there.

Criteria 2: (the strong criteria) A pair of vulnerable cells can be two cells that, taken together, posess all of the patterns of a given number's solution pattern set. The vulnerability here is if you can find an assertion that implies that neither of the cells contains the given number, then that assertion must be false (POM's general pattern exclusion rule)

Criteria 3: (the weak criteria) A pair of vulnerable cells can be two cells that have no two solution patterns for a number in common. The vulnerability here is if you can find an assertion which implies that the given number is placed in both of these cells, then that assertion must be false.

Pairings which satisfy both the strong and the weak criteria are doubly vulnerable. For these cases, in addition to the above, any assertion which merely implies that the pair has the same value must be false.

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. . . A B C . . .

. . . . . . . . .

. . . . . . . . .

. . . BC A . . . .

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. . . . C . . AB .

. . . . . AB . C .

In the diagram above, cells r5c4 and r9c6 would be strongly vulnerable. Cells r2c4 and r9c8 would be weakly vulnerable, and the r2c6-r8c8 pairing would be doubly vulnerable.

The advantage of these vulnerability points is that you are half way there. You just need to find the assertions that get you back to the start using whatever method you like best.

All is not skittles and beer. Many solution pattern sets do not have vulnerabilities. The simplest doesn't, and neither does the following.

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`. . . AB . CD . . .`

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. . . . . . . . .

. . . C . A . DB .

. . . . . . . . .

. . . D . B . AC .

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Nevertheless, here's hoping that these points lead to a few good starting places in solving your puzzles.