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David Bryant
Joined: 29 Jul 2005 Posts: 559 Location: Denver, Colorado

Posted: Mon Oct 03, 2005 8:11 pm Post subject: A very tough minimal sudoku 


I've been working through some of the 17clue Sudokus, just for practice. Interestingly, most of the 17clue Sudokus I've tried so far are fairly easy to solve. But some of them aren't so simple. Here's a very hard one.
Code: 
. . . 9 . . 8 4 .
5 . 1 . . . . . .
3 . . . . . . 9 .
. 4 . 2 . . 5 . .
. . . 7 6 . . . .
. . . . . . 3 . .
. . . . 3 1 . . .
. 6 . . . . . . 7
. . . . . . . . .

I didn't have to use any "advanced techniques" to get most of the way through this puzzle. I found it difficult during the early going because it wasn't easy to spot the next number that could be placed in the grid. After 52 moves (12 spots left open) I reached a position that wasn't easily resolved  but since there were only a few moves left at that point, brute force was enough to find the (unique) solution.
In case anyone else is interested in these "minimal" Sudoku puzzles, you can find a great number of them at http://www.csse.uwa.edu.au/~gordon/sudokumin.php. This one is #7768 from that collection. (Thanks to Someone_Somewhere for pointing this site out in an earlier post). dcb 

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someone_somewhere
Joined: 07 Aug 2005 Posts: 275 Location: Munich

Posted: Tue Oct 04, 2005 9:25 am Post subject: 


Hi David,
No need for brute force, an XYwing is solving it.
Tip:
2 not in r7c8, XYWing1 X=8 Y=9 in r5c1 X=8 Z=2 in r5c8 Y=9 Z=2 in r7c1
hope it fits the point you got stacked. If not I can supply you my solution.
see u, 

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David Bryant
Joined: 29 Jul 2005 Posts: 559 Location: Denver, Colorado

Posted: Tue Oct 04, 2005 11:57 am Post subject: 


You're right  that is the spot where I was stuck. I don't really know how to spot those "XYWing" combinations yet ... I guess I need more practice. dcb 

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someone_somewhere
Joined: 07 Aug 2005 Posts: 275 Location: Munich

Posted: Wed Oct 05, 2005 7:13 am Post subject: 


Hi David,
You want a couple of Sudoku that can be solved with XYwing?
see u, 

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David Bryant
Joined: 29 Jul 2005 Posts: 559 Location: Denver, Colorado

Posted: Thu Oct 06, 2005 10:33 pm Post subject: 


someone_somewhere wrote:  Hi David,
You want a couple of Sudoku that can be solved with XYwing?
see u, 
Yes, please  I'd like some more "XYWing" examples. You can send me email at davidbryant@att.net, or post a private message through this forum.
Thanks! dcb 

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Guest

Posted: Fri Oct 07, 2005 4:21 pm Post subject: Some Sudoku that need XYwing tehcnique 


Hi,
Here one that needs XYwing technique:
000010780
500009000
000000040
026000000
000600003
074080000
000003002
080040010
600500000
You will have to use some tehcniques like:
Row on 3x3 Block interaction
Column on 3x3 Block interaction
3x3 Block on Row/Column
Hidden Pair in Row
in Column
in 3x3 Block
Nacked Pair on Row
and of course:
XYwing
The above ones are sufficient.
good look!
see u, 

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Guest

Posted: Fri Oct 07, 2005 4:25 pm Post subject: 


Hi,
An other one that needs XYwing:
000010780
500009000
000000040
026000000
000600003
074080000
000003002
080040010
600500000
see u, 

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Guest

Posted: Fri Oct 07, 2005 4:28 pm Post subject: 


Hi,
one more:
014050000
000006008
002000000
000800002
005010900
600007000
000000100
700300000
000090540
see u, 

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Guest

Posted: Fri Oct 07, 2005 4:30 pm Post subject: 


Hi,
This one is a "Leckerbissen". You will need Xwing and XYwing.
010007030
000005000
062010840
370800000
008060200
000001089
089020350
000400000
050300060
see u, 

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Guest

Posted: Fri Oct 07, 2005 4:33 pm Post subject: 


Hi,
Now it should not be a problem to find the XYwing from this one:
900500362
253900810
601203950
069032085
100608039
300490076
530004698
890300741
010809523
see u, 

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Guest

Posted: Fri Oct 07, 2005 4:37 pm Post subject: 


Hi,
From this one:
020000000
000600003
074080000
000003002
080040010
600500000
000010780
500009000
000000040
you can eliminate 5 numbers using XYwing.
see u, 

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Guest

Posted: Fri Oct 07, 2005 4:40 pm Post subject: 


Hi,
In this one I could find also 1 XYwing:
000000021
500040000
000000070
000300600
000020500
010000000
600000203
003107000
000008000
see u, 

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Guest

Posted: Fri Oct 07, 2005 4:42 pm Post subject: 


Hi,
I got stucked after finding 33 numbers with this one:
400000001
080400000
000000000
000700850
001900000
200000000
030006080
600010400
XYwing solved it.
see u, 

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Guest

Posted: Fri Oct 07, 2005 4:46 pm Post subject: 


Ok, one with hidden pair, Xwing and XYwing:
500000000
400070006
908405000
010003090
005209600
040800020
000604503
700020004
000000009
By now, the coffee should be still hot after you have solved it!
see u, 

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Guest

Posted: Fri Oct 07, 2005 4:52 pm Post subject: XYwing of two kinds: on column and/or on row 


Hi,
By now you should have found out that there are two kind of XYwing:
on row and/or on column.
Hope that now we have enough example posted.
There is only one thing left: to find them!
good look & see u, 

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David Bryant
Joined: 29 Jul 2005 Posts: 559 Location: Denver, Colorado

Posted: Fri Oct 07, 2005 11:19 pm Post subject: Re: Some Sudoku that need XYwing tehcnique 


Someone_Somewhere wrote:  Hi,
Here one that needs XYwing technique:
000010780
500009000
...
see u, 
Thanks for all the examples. Since this topic is getting sort of long, I'm going to start a new topic, "XYWing Example #1," to discuss this puzzle.
Just for the record, I didn't have to use "XYWing" to solve this puzzle. But the first move was pretty hard to find ... after that it went fairly quickly. dcb 

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