## Notice

I am no longer posting new puzzles to this blog. For all of my Sudoku puzzles, old and new, please visit Sudoku in another section of this website. I will still create and offer new puzzles, in batches of a couple of hundred, once a week or so.

## Sudoku #11

Category: Sudoku
Thu, 30 Jun 2005, 22:53

In #10, I discussed basic Sudoku solving techniques. But occasionally, you'll meet a puzzle that needs more. In particular, all of the puzzles you see here need at least some of these advanced techniques.

The first is an extension of a basic technique. Previously, I mentioned looking for cells with only one possible value. But you can also look for pairs of cells in a group (row, column, or square) with the exact same two possibilities. Consider how this works. Let's say you have two cells, A and B, with the two possibilities {x,y}. If cell A had value x, then x would be eliminated from cell B, leaving it with value y (and vice versa). Thus, the possibilities x and y must exist in these two cells, and so, x and y can be eliminated from all other cells of this group.

You can also extend this technique to three cells in a group with the exact same three possibilities. This case doesn't come up much. But as before, it's also a situation that's easy to spot, so it's worth being aware of.

Next time, the second advanced solving technique.

Hans

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