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August 18, 2005

Comments

Larry

Hi Dan,
If I am understanding the X-Wings correctly then the only criteria for isolating a number using this method is that it must appear in each corner of a oblong.
If that is so, then in the first example you gave there is another oblong with 3 at each corner. That is the one bounded by 1,2 - 4,2 - 1,5 - 4,5.

Can you please either correct my misunderstanding of the method or tell me why the x-wings cannot work for the 3s in the oblong in column 1 and 5 but can with the one adjacent to it.
Thanks

sudokublog

Hi Larry -

It's not enough that the numbers form the corners of an oblong. In the first example, note the red vertical lines. These are there to indicate that 3 can only appear in the circled places in each of those columns. In columns 1 and 4, 3 can appear in more than 2 places. The X-Wings rule takes advantage of this knowledge of the columns in order to eliminate other values in the same rows.

Make sure you understand why the rule works before you try to apply it -- otherwise it's likely to lead you to an incorrect solution.

Steve Hauenstein

I have been working Sudoku puzzles for the last couple of months. I have yet to find one in a newspaper or book that I can't solve using only logic. I have found some on-line that I haven't been able to solve using logic, so I researched some of these complex pattern recognition methods. I was able to solve your X-wing example without using your X-wing process. Are there any puzzles that must use this type of reduction to solve logically, or is X-wing just a more efficient way to eliminate some numbers?

Stan Perkins

Thanks for the clearest examples
of X-wing I've been able to find.

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