20080320, 04:52  #1 
"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 
20080320, 05:14  #2 
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 :( 
20080320, 05:56  #3 
"Mark"
Feb 2003
Sydney
3×191 Posts 
The cell in the lowerleft 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 lowerleftcorner cell can't be 3, leaving just one possibility. There's probably other ways to get to the solution. 
20080320, 11:32  #4 
Dec 2005
11000100_{2} Posts 

20080320, 19:20  #5 
1976 Toyota Corona years forever!
"Wayne"
Nov 2006
Saskatchewan, Canada
5^{2}×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 .... 
20080321, 01:10  #6 
Aug 2003
Snicker, AL
3C0_{16} 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 
20080321, 01:43  #7 
Aug 2003
Snicker, AL
2^{6}×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 
20080321, 04:22  #8 
"Juan Tutors"
Mar 2004
1073_{8} 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.

20080321, 15:17  #9 
"Lucan"
Dec 2006
England
2·3·13·83 Posts 
Brute force (and ignorance)?
Last fiddled with by davieddy on 20080321 at 15:50 
20080321, 15:20  #10  
"Lucan"
Dec 2006
England
6474_{10} Posts 
Quote:
Only one other way? What is it? 

20080321, 15:31  #11 
Undefined
"The unspeakable one"
Jun 2006
My evil lair
5·7·191 Posts 

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