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David Bryant Gold Member

Joined: 20 Jan 2006 Posts: 86 Location: Denver, Colorado
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Posted: Fri Jan 27, 2006 9:16 pm Post subject: 27 Jan 2006 Nightmare |
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I'm not sure this is a new technique. But in the course of solving today's "Nightmare" I hit an interesting formation -- I'm not sure I've ever encountered one exactly like it before.
With 45 cells resolved and 36 still to go the puzzle looked like this (I had only made the obvious moves up to this point, plus I had used some simple coloring to eliminate one "6" [at r8c5] and one "8" [at r5c8] from the matrix).
Code: | 6 8 2 9 5 7 4 9 1
1 4 3 8 26 9 5 26 7
5 7 9 124 1246 146 68 268 3
3 9 78 6 78 5 2 1 4
278 12 6 147 3 14 9 57 58
4 5 178 179 1789 2 78 3 6
278 26 5 2479 24679 3 1 478 89
27 3 4 1279 1279 8 67 567 59
9 16 178 5 1467 146 3 4678 2 |
I wasn't seeing anything obvious and was focusing on the simple pairs, looking for a regular XY-Wing, when I spotted an unusual pattern near the bottom right corner. It's sort of a "crippled" XY-Wing.
r7c9 = 8 ==> r5c9 = 5 ==> r5c8 = 7 ==> no "7"s in rest of col 8
r7c9 = 9 ==> r8c9 = 5 ==> {6, 7} pair in row 8 ==> r7c8, r9c8 <> 7
So either way there can't be a "7" at r7c8 or r9c8 -- this leaves a naked quad and a "hidden" pair {5, 7} in column 8, and the "6" in r8c7 is revealed as unique in row 8 -- the rest of the puzzle just falls apart!
I'm sure there are other ways to solve this puzzle -- I don't think it's as hard as a typical "Nightmare." But I sort of like this way. It's cute. dcb |
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Myth Jellies Hooked

Joined: 04 Apr 2006 Posts: 42
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Posted: Tue Apr 04, 2006 7:27 am Post subject: |
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You can get this solution out of a nice grouped candidate chain as follows...
Code: |
6 8 2 9 5 7 4 9 1
1 4 3 8 26 9 5 26 7
5 7 9 124 1246 146 68 268 3
3 9 78 6 78 5 2 1 4
278 12 6 147 3 14 9 *57 *58
4 5 178 179 1789 2 78 3 6
278 26 5 2479 24679 3 1 478 *89
27 3 4 1279 1279 8 *67 *567 *59
9 16 178 5 1467 146 3 4678 2
[7==5]-[5==8]-[8==9]-[9==5]-[5=(6&7)]
[r5c8] [r5c9] [r7c9] [r8c9] [ r8c78 ]
chain implies that all cells seeing r5c8, r8c7, & r8c8 cannot be 7
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Ron Moore Addict

Joined: 13 Aug 2006 Posts: 72 Location: New Mexico
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Posted: Sun Aug 13, 2006 3:56 am Post subject: An XYZ wing with two eliminations |
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I just joined the forum, so I'm aware that this is very old business. I wouldn't have commented except that the solving guide states that no XYZ wing resulting in two eliminations had been discovered in practice. Well, here is one. r8c8 (pivot), r5c8, and r8c7 form the XYZ wing pattern (no need to consider column 9), which, as the previous posts have concluded, eliminates 7 from r79c8. |
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David Bryant Gold Member

Joined: 20 Jan 2006 Posts: 86 Location: Denver, Colorado
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Posted: Thu Aug 17, 2006 10:45 pm Post subject: Astute observation |
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That's an astute observation, Ron. I'm not too good with names for some of these newer "techniques" -- I tend to rely on logic & first principles. Thanks for pointing out that this is an "XYZ-Wing" pattern.
Maybe we can get Ruud to update the solving guide. Here's an example of the double elimination XYZ-Wing in one of the early "nightmares" he published! dcb |
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Ruud Site Owner

Joined: 30 Dec 2005 Posts: 601
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Posted: Thu Aug 17, 2006 11:45 pm Post subject: |
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I've updated the guide. A pointer to this Nightmare has been added to the XYZ-Wing topic.
Thanks, Ron. This is the second time in a row that others have found examples I was looking for in my own puzzles.
Ruud |
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Ron Moore Addict

Joined: 13 Aug 2006 Posts: 72 Location: New Mexico
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Posted: Fri Aug 18, 2006 4:09 am Post subject: Another Double Elimination XYZ wing |
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As it happens, while browsing through the site shortly after my post above, I came across another example which Myth Jellies points out in this post: http://sudocue.net/forum/viewtopic.php?t=194.
By the way, David, I've seen many fine posts by you and you're familiar with some techniques that are new to me.
Ron |
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