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.
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:
- N = 2: Hidden Pair
- N = 3: Hidden Triplet
- N = 4: Hidden Quad
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: 12/7/2018 7:18:01 AM CST