Is this puzzle solveable with logic?

Advanced methods and approaches for solving Sudoku puzzles

Postby RodHagglund » Wed Dec 28, 2005 5:04 am

Actually there's a trick to solve this one in fewer steps. After you use the naked pair in r3c5 and r3c6 to remove a 5 from r1c3, you can use the uniqueness condition to exclude the possibility of a 1 in r5c3.

A 1 placed there would remove the 1 from r4c1, and also place a 3 in r1c3, which in turn would place a 5 in r6c3, removing the 5 from r6c5. This would leave the numbers 2 and 3 as possible candidates for r4c1, r6c1, r4c5 and r6c5. Since the 2's and 3's can be placed in those cells in two different ways without affecting the reasoning for any other cell, there would be at least two solutions to the sudoku; the uniqueness assertion is that if we trust this is a valid sudoku and has only one solution, we can conclude that there can't be a 1 in r4c1.

That leaves only r1c3 as a place for a 1 in column 3, and from there the puzzle solves.
Rod Hagglund
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Postby Jeff » Wed Dec 28, 2005 5:55 am

9X9 wrote:Although I'm on a sabbatical, I risk losing the BUG plot if I don't pitch in now. Although I can grasp part of it, the long thread developing and elucidating BUG, contributed to by genuine experts like yourself, is mainly just so much "number soup" for me.

Any chance of a succint and crystal clear "dummies" version?

Hi 9x9, You can't be on a sabbatical if you can get access to a computer. Anyway, here and here are good places to start with.
Jeff
 
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Postby 9X9 » Wed Dec 28, 2005 11:26 pm

Jeff

Great - that's just the ticket. I hadn't read all of it before because a) I didn't need to and b) it seemed like very heavy stuff. I worked through the totality of angus and simes at the start of my Sudoku journey and then happily settled at the X-Wing level, since that was all I needed to crack the Pappocom Very Hards and all I thought I'd ever need.

Now I'd like to attack greater heights, once the sabbatical is over (smuggled the laptop into the cave when no one was looking).
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