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keith
Joined: 19 Sep 2005 Posts: 3192 Location: near Detroit, Michigan, USA

Posted: Sat Jun 10, 2006 11:30 pm Post subject: Saturday, June 10 v. hard 


Sam's got us this time!
So far as I can see, the only (reasonable) way to solve this one is with a difficult to spot Xwing. No Unique Rectangles here!
Keith 

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Marty R.
Joined: 12 Feb 2006 Posts: 5275 Location: Rochester, NY, USA

Posted: Sun Jun 11, 2006 5:38 am Post subject: 


Quote:  So far as I can see, the only (reasonable) way to solve this one is with a difficult to spot Xwing. 
I don't know if my way was reasonable or not, but after the basic stuff, I was looking at a rectangle, a Type 3, I believe, but not one that could be solved directly with basic rectangle logic.
However, in the same box as the two corners, there was a third cell containing only the two deadly candidates. I started a chain with the lower number first, and that solved the entire puzzle. 

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ulysses
Joined: 30 Jul 2005 Posts: 3

Posted: Sun Jun 11, 2006 1:23 pm Post subject: 


What's puzzling me about this sudoku is that it appears to have two solutions. I can get the given solution but equally I can get the following solution:
594712638/768953214/123468795
472689531/386125479/951374862
815237946/639541827/247896153
These are all rows starting at the top and working downwards.
I got to a point where there was a subset of numbers that appeared to be self contained. I took a square with only 2 possible solutions and tried each in turn. Once produced the given solution and the other produced the above solution. I can see nothing wrong with it.
I should also say I'm in the habit of missing obvious errors so I would be grateful (if there is an error) if someone could point it out to me! 

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jabejochke
Joined: 16 Mar 2006 Posts: 21 Location: Reading

Posted: Sun Jun 11, 2006 2:02 pm Post subject: 


Code: 
5 9 2348 7 12 24 6 1238 28
17 6 28 9 5 3 278 128 4
17 24 234 46 126 8 237 9 5
234 7 246 346 8 9 5 23 1
234 8 246 1 26 5 234 7 9
9 5 1 34 7 24 2348 6 28
8 1 5 2 3 7 9 4 6
6 3 9 5 4 1 28 28 7
24 24 7 8 9 6 1 5 3 
This is point I reached when I started looking for xwings, unique rectangles etc.
Found the xwing of 2's at R16C69 and a rectangle at R28C78. Took the Xwing path and went to solution; however, could not do anything with the unique rectangle. Can the solution be gotten using the UR as the starting point.
Appreciate any insights.
Thanks,
Jabejochke
PS Please excuse the wiggly picture  I haven't figured out how to use the Code: function. 

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ulysses
Joined: 30 Jul 2005 Posts: 3

Posted: Sun Jun 11, 2006 3:13 pm Post subject: 


That's the point I got to. I found if I entered 4 in the extreme bottom left square, I got the given answer. However, if I entered 2 (other other possibility) I got the alternative solution I posted, which also appears to be OK. 

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Marty R.
Joined: 12 Feb 2006 Posts: 5275 Location: Rochester, NY, USA

Posted: Sun Jun 11, 2006 4:05 pm Post subject: 


Code: 
5 9 2348 7 12 24 6 1238 28
17 6 28 9 5 3 278 128 4
17 24 234 46 126 8 237 9 5
234 7 246 346 8 9 5 23 1
234 8 246 134 7 24 2348 6 28
8 1 5 2 3 7 9 4 6
6 3 9 5 4 1 28 28 7
24 24 7 8 9 6 1 5 3 
Quote:  Found the xwing of 2's at R16C69 and a rectangle at R28C78. Took the Xwing path and went to solution; however, could not do anything with the unique rectangle. Can the solution be gotten using the UR as the starting point. 
Yes and no. I don't believe anything can be done directly with the UR, but with r1c9 containing the 28, that's a good opportunity for starting a forcing chain. That's what I did and the "2" solved the puzzle.
Quote:  PS Please excuse the wiggly picture  I haven't figured out how to use the Code: function. 
As recommended to me here, I type my grids in Notepad, but it should work OK here too. The key is having the same number of characters, including spaces, for every cell. Look at the cell with the most candidates and add one or two. In this case four is the most candidates in a cell, so I add one, thus every cell will have five characters. If the cell is solved, I type four spaces after the single digit. If it has two candidates, type three spaces, and so on such that the the number of candidates plus spaces equals five, in this case. I copied your grid to Notepad and it took about one minute to align it via typing spaces and/or backspacing. 

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keith
Joined: 19 Sep 2005 Posts: 3192 Location: near Detroit, Michigan, USA

Posted: Sun Jun 11, 2006 4:10 pm Post subject: Invalid solution! 


You posted:
Code: 
594712638
768953214
123468795
472689531
386125479
951374862
815237946
639541827
247896153

which has two 8's in C7, and two 3's in C8.
Keith 

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ulysses
Joined: 30 Jul 2005 Posts: 3

Posted: Sun Jun 11, 2006 4:45 pm Post subject: 


Many thanks. As I said, I have a tendency to make errors which are obvious once they are pointed out! 

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gf1701
Joined: 27 May 2006 Posts: 4

Posted: Wed Jun 14, 2006 6:26 am Post subject: 


I've read these postings but I don't understand the language. What is an Xwing? What is a deadly candidate? What does the notation R16C69 refer to (obviously not Row 16, Column 69!)?
Gary
P.S. When I tried the Hint feature on this puzzle, at the point I'd reached, nothing happened; first time I've seen that. 

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keith
Joined: 19 Sep 2005 Posts: 3192 Location: near Detroit, Michigan, USA


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gf1701
Joined: 27 May 2006 Posts: 4

Posted: Mon Jun 19, 2006 3:27 am Post subject: 


Thanks for the links. I think http://angusj.com/sudoku/hints.php is the best place to go for an explanation of terms. But I still don't know what R16C69 and R28C78 refer to. Without that information, I can't interpret the posting above that points to the puzzle's solution. 

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Marty R.
Joined: 12 Feb 2006 Posts: 5275 Location: Rochester, NY, USA

Posted: Mon Jun 19, 2006 5:04 am Post subject: 


Quote:  But I still don't know what R16C69 and R28C78 refer to. 
R16C69 = rows 1 and 6, columns 6 and 9.
R28C78 = rows 2 and 8, columns 7 and 8. 

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