Decabit Software
Hidden Subset

If in a unit, there is a set of N candidates that only appears in N cells, then you can remove all other candidates from those cells.

Consider the sudoku below.

1
2
6
1 2
3
4 5
3
4 5 6
3
4 5 6
3
4 5
1 2 3
4 5 6
1 2 3
4 5 6
7 8 9

In the center block, the candidate set {1, 2} only appears in two cells, r5c5 and r5c6. Therefore, we can remove all other candidates from those cells.

Common names for values of N:

Some of you may notice that Naked Subset and Hidden Subset techniques are very similar. In fact, the sudoku above could be identically simplified using the Naked Subset technique where N=4 using the cells r4c4, r4c5, r4c6, and r5c4. It can be shown that in a unit with X unsolved cells, if Naked Subset where N=A can simplify the sudoku, then Hidden Subset where N=X-A will identically simplify the sudoku.
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Last Updated: 8/5/2017 12:27:29 PM CST