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Naked Pairs and Triples

The naked pairs technique for Killer puzzles is adapted from the technique for regular Sudoku puzzles (you can read about this technique in that context here). The rule is: if two digits can only appear in two cells in a group or a cage, then those candidates can be eliminated from the rest of the group or cage. Here is an example of this technique:

In this example, we can remove the candidates "2" and "6" from the cells highlighted in red. The cells highlighted in orange can both only contain "2" or "6". Because they are in the same group, if one is a "2" then the other is a "6" and vice-versa. This means that both "2" and "6" must appear in the cells highlighted in orange, and therefore can't occur in the red cells (which share a group with the orange ones).

Here is an example of this technique used in a cage instead of a group:

In this case, the candidates "3" and "4" can be removed from the cell highlighted in red. This is because the orange cells must, between them, contain both "3" and "4", and the red cell shares a cage with them, so can contain neither.

It is also possible to find naked triples where 3 cells can only take 3 possible values. Here is an example of this:

In this example, the candidates 1 and 2 can be removed from the cells highlighted in red. This is because the orange cells must contain the digits 1, 2 and 5, so the red cells, sharing a group with the orange ones, cannot.

The naked n-tuple technique is the most advanced of the regular Sudoku techniques that Puzzle Tiger uses in Killer puzzles. All techniques used in regular Sudoku puzzles can be used without modification in Killer puzzles. Puzzle Tiger, however, doesn't look for the more advanced regular Sudoku techniques, because this detracts from the fun of the Killer puzzle. If you do happen to spot a hidden pair (or an X-wing or any of the more advanced techniques used in regular Sudoku) you can use it, but these techniques are never needed to solve Killer puzzles generated by Puzzle Tiger.

Copyright © Adam A. Brown, 2006, All Rights Reserved. www.sudokutiger.com