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 2008-03-20, 04:52 #1 JuanTutors     "Juan Tutors" Mar 2004 571 Posts Sudoku puzzle help I'm having a hell of a time with this sudoku puzzle. I've never gotten to a point that a puzzle would take me even close to an hour, except for this puzzle. What I would like from you puzzle geniuses: 1. I'm pretty sure there are no mistakes. I checked it over. But If I made an error, tell me what it is. 2. Otherwise, find a number and tell me how you found it. The black numbers are the numbers the puzzle came with. The grey numbers are the numbers I entered. http://www.ourspacer.com/misc/sudoku.jpg
 2008-03-20, 05:14 #2 retina Undefined     "The unspeakable one" Jun 2006 My evil lair 5·7·191 Posts My solver program shows it is solvable: 358617924 267489351 941325786 482753169 736891245 519264873 875936412 123548697 694172538 I have no idea how it solved it though :(
 2008-03-20, 05:56 #3 markr     "Mark" Feb 2003 Sydney 3×191 Posts The cell in the lower-left corner is 6. From there it falls out. I got to where you did no problem, then was lucky with a method I'm just starting to get the hang of. If you look at the five cells highlighted blue, they all have just two possible numbers, and form a chain (3,7) (7,1) (1,3) (3,1) (1,3). If the first cell is not 3, it's 7, so the next cell in the chain is 1, and so on until the last one is 3. Either way, at least one end of the chain is a 3. So the lower-left-corner cell can't be 3, leaving just one possibility. There's probably other ways to get to the solution. Attached Thumbnails
 2008-03-20, 11:32 #4 Kees     Dec 2005 110001002 Posts
 2008-03-20, 19:20 #5 petrw1 1976 Toyota Corona years forever!     "Wayne" Nov 2006 Saskatchewan, Canada 52×211 Posts YES.... this is the one I use when all else fails. http://www.scanraid.com/sudoku.htm It is by far the smartest "I have found" so far .... meaning I haven't checked out every one (by no means) but when I found this one I was impressed. It has many VERY advanced techniques and as a bonus it has links to describe these techniques ... AND ... if all else fails it has links to another very advanced site complete with a forum. Go crazy ....
 2008-03-21, 01:10 #6 Fusion_power     Aug 2003 Snicker, AL 3C016 Posts When I first saw this one I hoped it would be really hard. It took about 20 minutes. It is VERY susceptible to chain analysis. DarJones
 2008-03-21, 01:43 #7 Fusion_power     Aug 2003 Snicker, AL 26×3×5 Posts Just so an explanation is given for the problem posed, here is the way it should be analyzed. Look at cell A5. This is the cell at top center. Only two possibilities can fit, either 1 or 7. Regardless of what value you put in this cell, the only value that can possibly fit in cell I1 is 6. This is the cell at the bottom left corner. You can find this referred to as a "forced chain". There is one other way to analyze this but the above is the simplest. DarJones
 2008-03-21, 04:22 #8 JuanTutors     "Juan Tutors" Mar 2004 10738 Posts I'll look up this article on sudoku. I like coming up w/ the tricks myself, but I never learned this "forced chain" concept. I've used it before, but it never seemed so viable. Maybe there's a smart way of knowing where to start looking? I don't use notes.
2008-03-21, 15:17   #9
davieddy

"Lucan"
Dec 2006
England

2·3·13·83 Posts

Quote:
 Originally Posted by retina My solver program shows it is solvable: I have no idea how it solved it though :(
Brute force (and ignorance)?

Last fiddled with by davieddy on 2008-03-21 at 15:50

2008-03-21, 15:20   #10
davieddy

"Lucan"
Dec 2006
England

647410 Posts

Quote:
 Originally Posted by Fusion_power Just so an explanation is given for the problem posed, here is the way it should be analyzed. There is one other way to analyze this but the above is the simplest. DarJones
See Markr's post for a very clear description of the "chain".

Only one other way? What is it?

2008-03-21, 15:31   #11
retina
Undefined

"The unspeakable one"
Jun 2006
My evil lair

5·7·191 Posts

Quote:
 Originally Posted by davieddy Brute force?
I expect so. This computer is a brute, if I drop it on my toe it won't say sorry.

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