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Very Hard 13-01-2009

 
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leocar



Joined: 04 Mar 2010
Posts: 12

PostPosted: Thu Mar 04, 2010 5:46 am    Post subject: Very Hard 13-01-2009 Reply with quote

Hi, I'm just learning how to complete the very hard sudoku's and I'm confussed with the x-y wings. I have attached a code from 13-01-2009 VH.

I thought there was an x-y wing 567 (R6C1, R5C3, R4C8) removing 5 from R4C1 but that didn't work. Instead I used 679 (R2C1, R4C2, R5C3) removing 9 from the same cell to finish it. Is the second method correct or just luck? If it is correct, why is the first one incorrect? Thanks
Leo

Code:

+-------------+--------------+-----------+
| 2  5   3679 | 4679 367 347 | 8   37  1 |
| 79 679 3679 | 1    8   2   | 39  4   5 |
| 4  1   8    | 79   5   37  | 239 237 6 |
+-------------+--------------+-----------+
| 5  69  2    | 3    1   8   | 7   6   4 |
| 3  8   67   | 45   67  45  | 1   9   2 |
| 7  4   1    | 567  2   9   | 35  356 8 |
+-------------+--------------+-----------+
| 8  3   5    | 2    9   6   | 4   1   7 |
| 6  79  79   | 8    4   1   | 25  25  3 |
| 1  2   4    | 57   37  357 | 6   8   9 |
+-------------+--------------+-----------+
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Marty R.



Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA

PostPosted: Thu Mar 04, 2010 6:09 am    Post subject: Reply with quote

Leo, I edited your post to get rid of the long string so the whole thing could be seen on the screen without scrolling.

Your 679 XY-Wing isn't a Wing because the pivot (69) doesn't see the other two cells. The rule for an XY-Wing is to start with a cell we'll call XY. That cell must see both an XZ and YZ cell. Then any cell seeing both XZ and YZ cannot contain Z.

However, there is an XY-Wing involving 679 in r5c3, r4c2 and r8c3. R8c2 sees both the XZ (69) and YZ (79), thus 9 is removed from r8c2.

There can't be anything involving r6c1 since that cell is solved.

Welcome to the forum and feel free to ask all the questions you'd like. There are lots of folks happy to help out a new member.
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leocar



Joined: 04 Mar 2010
Posts: 12

PostPosted: Fri Mar 05, 2010 7:22 am    Post subject: Reply with quote

Thank you for that, for some reason I was using the cell I was trying to eliminate from as the pivot! It's much clearer now.

Cheers
Leo
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