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2 July VH
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cgordon



Joined: 04 May 2007
Posts: 769
Location: ontario, canada

PostPosted: Thu Jul 05, 2007 6:27 pm    Post subject: Reply with quote

Apologies if I've missed something here. But it seems that here we have traced a link from one 19 to another 19 such that IF the first cell is a 1 then the other has to be a 9 - and vice-versa. But surely a good technique should tell us which is the 1 and which is the 9 - right off the bat. Otherwise why not just go to the first 19 and say "let x = 1"and see where it goes. An assumption is an assumption. Again, apologies if I've missed the point.
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Thu Jul 05, 2007 6:44 pm    Post subject: Reply with quote

The point is:

Either R3C6 and / or R7C2 is <9>. Thus R3C2 cannot be <9>. Which causes a pair <16> in R3, which solves the puzzle because R3C6 is <9>, etc.

Keith
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Asellus



Joined: 05 Jun 2007
Posts: 865
Location: Sonoma County, CA, USA

PostPosted: Thu Jul 05, 2007 7:31 pm    Post subject: Ponder This Reply with quote

I notice that a W-Wing could serve as a "Bridge" for a color wrap. The idea is of no use in the two examples in this thread, and I don't even know if such an occurrence exists. But, it would be neat if someone found one.

To clarify, borrowing Keith's schematic for the W-Wing:

<XY> -- <X-Strong Link-X> -- <XY>

For purposes of the W-Wing, we focus on the fact that one or both of the bivalue cells must be <Y>.

But, the converse statement that the bivalue cells cannot both be <X> qualifies them to serve as a Bridge for a Color Wrap on <X>.

I'll be on the lookout for one.
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cgordon



Joined: 04 May 2007
Posts: 769
Location: ontario, canada

PostPosted: Thu Jul 05, 2007 7:39 pm    Post subject: Reply with quote

Thanks - but I'll stick to the simple xy wing that was originally needed to solve this one. The above "W wing variations" seem like using a sledge-hammer to crack a walnut. Definitely wouldn't agree they are easier than xy wings or skyscrapers.
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ravel



Joined: 21 Apr 2006
Posts: 536

PostPosted: Thu Jul 05, 2007 7:49 pm    Post subject: Reply with quote

keith wrote:
jLo said:
Quote:
In the July 2 puzzle there is a W-Wing with the pair "19"
So far as I can see, correct, but not so simple.
Hm. did you miss the strong link for 1 in column 1 ?
Code:
+--------------+-------------+-------------+
| 7   128 3   | 15  6   4   | 28  9   258 |
| 29  28  94   | 359 7   359 | 6   1   258 |
|#16  169$ 5   | 2   8   19@ | 4   3   7   |
+-------------+-------------+-------------+
| 4   7    9   | 6   5   23  | 1   28  238 |
| 38  5    28  | 13  4   12  | 9   7   6   |
| 36  26   1   | 7   9   8   | 23  5   4   |
+--------------+-------------+-------------+
| 5   19@  28  | 4   3   6   | 7   28  19  |
|#18  4    7   | 59  2   59  | 38  6   138 |
| 29  3    6   | 8   1   7   | 5   4   29  |
+--------------+-------------+-------------+
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Asellus



Joined: 05 Jun 2007
Posts: 865
Location: Sonoma County, CA, USA

PostPosted: Thu Jul 05, 2007 8:34 pm    Post subject: Reply with quote

Ravel,

I think the point was that there are many different ways to find the needed strong link. (There is also one in Row 1.)

From the "Y-Wing" link in your Other Puzzles post today, it is clear that the W-Wing is one example of gsf's more general category of "Y-Wing" (as are X-Wings, XY-Wings and almost all the commonly named techniques!). However, the "Y-Wing" discussion goes right over the head of most of us casual sudoku types... or at least this one! Still, gsf clearly described the "W-Wing" back in March and recommended it as an easy to spot method.
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ravel



Joined: 21 Apr 2006
Posts: 536

PostPosted: Fri Jul 06, 2007 7:52 am    Post subject: Reply with quote

Asellus wrote:
I think the point was that there are many different ways to find the needed strong link.
I see, sorry, i did not read all of the thread.
Quote:
From the "Y-Wing" link in your Other Puzzles post today, it is clear that the W-Wing is one example of gsf's more general category of "Y-Wing" (as are X-Wings, XY-Wings and almost all the commonly named techniques!). However, the "Y-Wing" discussion goes right over the head of most of us casual sudoku types... or at least this one! Still, gsf clearly described the "W-Wing" back in March and recommended it as an easy to spot method.
As i see it, Steve K first described it as an example of his "Y Wing styles", but i am not sure (when you search for "Y Wing", you get all threads with wings).
I only think to remember, that rep'nA used W-Wings calling them Y Wings (probably because there was no own name for this type of "Y Wing style"). Btw all his posts mysteriously vanished and he is now posting as "re'born" Smile
I also think, that it is rarely known, that W-Wings are rather common in puzzles and (as Marty demonstrated again) often easier to spot that xy-wings. So it is worth to be looked at as an own technique.
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Fri Jul 06, 2007 8:31 am    Post subject: Reply with quote

ravel wrote:
keith wrote:
jLo said:
Quote:
In the July 2 puzzle there is a W-Wing with the pair "19"
So far as I can see, correct, but not so simple.
Hm. did you miss the strong link for 1 in column 1 ?


Yes I did! And the one pointed out be Asellus!

I was looking for links which line up in a row or column, and neglected to look at those in the same box. A franken W-wing, I suppose.

Best wishes,
Keith
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