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Feb 2 VH

 
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TerenceF



Joined: 21 Dec 2007
Posts: 19
Location: Takapuna, NZ

PostPosted: Wed Feb 01, 2012 9:22 pm    Post subject: Feb 2 VH Reply with quote

With my new found XY-wing hunting skills I spotted a 5-8-9 wing pivoted at r8c7 which eliminated the 5 from r1c8. But that didn't get me much further.
Sad

Code:

+------------+-------+-----------+
| 1   59 2   | 7 6 3 | 589 589 4 |
| 8   6  3   | 9 4 5 | 1   7   2 |
| 57  4  579 | 8 2 1 | 59  6   3 |
+------------+-------+-----------+
| 56  7  56  | 1 9 4 | 3   2   8 |
| 3   1  8   | 2 5 6 | 7   4   9 |
| 9   2  4   | 3 8 7 | 6   1   5 |
+------------+-------+-----------+
| 2   3  59  | 6 1 8 | 4   59  7 |
| 4   89 1   | 5 7 2 | 89  3   6 |
| 567 58 567 | 4 3 9 | 2   58  1 |
+------------+-------+-----------+

But there is also a 589 combination pivoted on r8c2 which in conjunction with r1c2 and r8c7 and r1c7 means that r3c7 cannot be 9 (or must be 5 depending on which number you focus on), after which it drops out.

What is that combination called again??
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arkietech



Joined: 31 Jul 2008
Posts: 1728
Location: Northwest Arkansas USA

PostPosted: Wed Feb 01, 2012 10:27 pm    Post subject: Re: Feb 2 VH Reply with quote

TerenceF wrote:
With my new found XY-wing hunting skills I spotted a 5-8-9 wing pivoted at r8c7 which eliminated the 5 from r1c8. But that didn't get me much further.
Sad

Code:

+------------+-------+-----------+
| 1   59 2   | 7 6 3 | 589 589 4 |
| 8   6  3   | 9 4 5 | 1   7   2 |
| 57  4  579 | 8 2 1 | 59  6   3 |
+------------+-------+-----------+
| 56  7  56  | 1 9 4 | 3   2   8 |
| 3   1  8   | 2 5 6 | 7   4   9 |
| 9   2  4   | 3 8 7 | 6   1   5 |
+------------+-------+-----------+
| 2   3  59  | 6 1 8 | 4   59  7 |
| 4   89 1   | 5 7 2 | 89  3   6 |
| 567 58 567 | 4 3 9 | 2   58  1 |
+------------+-------+-----------+

But there is also a 589 combination pivoted on r8c2 which in conjunction with r1c2 and r8c7 and r1c7 means that r3c7 cannot be 9 (or must be 5 depending on which number you focus on), after which it drops out.

What is that combination called again??

The 9's in row 3 and 7 column 3, 7 and 8 form what is called a skyscraper.
either r3c7 or r7c8 must be a 9 =>(therefore) r8c7 and r1c8 cannot be a 9
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Clement



Joined: 24 Apr 2006
Posts: 684
Location: Dar es Salaam Tanzania

PostPosted: Wed Feb 01, 2012 11:20 pm    Post subject: Feb 2 VH Reply with quote

I have the following Grid after the Basics
Code:

+--------------+----------+-------------+
| 1    59 259  | 7  6 3   | 589   589 4 |
| 8    6  3    | 9  4 5   | 1     7   2 |
| 57   4  579  | 8  2 1   | 59    6   3 |
+--------------+----------+-------------+
| 56   7  56   | 1  9 4   | 3     2   8 |
| 3    1  8    | 26 5 26  | 7     4   9 |
| 9    2  4    | 3  8 7   | 6     1   5 |
+--------------+----------+-------------+
| 25   3  259  | 26 1 268 | 24589 589 7 |
| 4    89 1    | 5  7 28  | 289   3   6 |
| 2567 58 2567 | 4  3 9   | 258   58  1 |
+--------------+----------+-------------+
Type 1 UR 26 in r57; r7c6=8 which opens a W-Wing 59 stongly linked by 9 in col 8; r34c3<>5 which solves it.
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Marty R.



Joined: 12 Feb 2006
Posts: 5165
Location: Rochester, NY, USA

PostPosted: Thu Feb 02, 2012 12:57 am    Post subject: Re: Feb 2 VH Reply with quote

TerenceF wrote:
With my new found XY-wing hunting skills I spotted a 5-8-9 wing pivoted at r8c7 which eliminated the 5 from r1c8. But that didn't get me much further.
Sad

Code:

+------------+-------+-----------+
| 1   59 2   | 7 6 3 | 589 589 4 |
| 8   6  3   | 9 4 5 | 1   7   2 |
| 57  4  579 | 8 2 1 | 59  6   3 |
+------------+-------+-----------+
| 56  7  56  | 1 9 4 | 3   2   8 |
| 3   1  8   | 2 5 6 | 7   4   9 |
| 9   2  4   | 3 8 7 | 6   1   5 |
+------------+-------+-----------+
| 2   3  59  | 6 1 8 | 4   59  7 |
| 4   89 1   | 5 7 2 | 89  3   6 |
| 567 58 567 | 4 3 9 | 2   58  1 |
+------------+-------+-----------+

But there is also a 589 combination pivoted on r8c2 which in conjunction with r1c2 and r8c7 and r1c7 means that r3c7 cannot be 9 (or must be 5 depending on which number you focus on), after which it drops out.

What is that combination called again??

Terence,

You can go further on basics. The 5 from r1c8 is deleted because of the 5s in box 9 and 5s can be eliminated in box 7 because of the 589 triple in that box. There are at least two XY-Wings that solve it. 598 pivoted in r1c2 and 895 pivoted in r1c8. There are also several non-VH moves that can be used.
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TerenceF



Joined: 21 Dec 2007
Posts: 19
Location: Takapuna, NZ

PostPosted: Thu Feb 02, 2012 4:51 am    Post subject: Reply with quote

Thanks Marty, voice of reason again. Smile

Interesting that the 5 from r1c8 was eliminated by the wing I spotted, and yet you saw a better more basic move that gives the same result.

I like to think that I had also spotted the 589 triple in Box 7 the first time I solved the puzzle: but in order to post in this forum, I tend to do the puzzle once, note roughly where the key solving move appeared, and then go through the puzzle again sticking to only the most basic moves until the "unlock" pattern emerges: then take a snapshot and confirm that the puzzle is solvable by basic moves after that.

I think Clement does the same: the grid he posted has some basic moves still available, but his solution is more elegant for that, because it reaches the nub of the puzzle more quickly.

That said, almost every time I get stuck and come to this forum to get the solution I find that the grid posted differs from where I got to by just a couple of basic moves - so perhaps the answer is to devote more effort to the basics before looking for fancy patterns.

As you observed in your last post, this comes under the category of grunt work - whereas it is much more satisfying if a complex pattern just leaps off the page at you. The psychology of solving these is quite intriguing.

If you read this far, thanks for reading, end of ramble.
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Marty R.



Joined: 12 Feb 2006
Posts: 5165
Location: Rochester, NY, USA

PostPosted: Thu Feb 02, 2012 6:18 am    Post subject: Reply with quote

Just for the sake of posting, if the basics were carried out to the fullest, another move is made available, a BUG+2. In case you weren't aware, these VH puzzles can be solved by using just X-, XY-, and XYZ-Wings.

You'll note that both Dan and Clement used other moves, which is fine. There are many ways that the puzzles can be solved. As for me personally, I try to solve them using the standard VH moves, just for the sake of a small challenge, but there's no right or wrong way.

But I do think it's always a good idea to take the basics to the max because often that is essential to finding a reasonable solution.
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Clement



Joined: 24 Apr 2006
Posts: 684
Location: Dar es Salaam Tanzania

PostPosted: Thu Feb 02, 2012 6:36 am    Post subject: Feb 2 VH Reply with quote

Your are quite right Marty. I missed the lonely candidate 2 in row 1. For that matter the UR is not there after the complete basics.
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hughwill



Joined: 05 Apr 2010
Posts: 217
Location: Birmingham UK

PostPosted: Thu Feb 02, 2012 10:44 am    Post subject: Feb 2 VH Reply with quote

Code:

+-----------+-------+----------+
| 1  59 2   | 7 6 3 | 589 89 4 |
| 8  6  3   | 9 4 5 | 1   7  2 |
| 57 4  579 | 8 2 1 | 59  6  3 |
+-----------+-------+----------+
| 56 7  56  | 1 9 4 | 3   2  8 |
| 3  1  8   | 2 5 6 | 7   4  9 |
| 9  2  4   | 3 8 7 | 6   1  5 |
+-----------+-------+----------+
| 2  3  59  | 6 1 8 | 4   59 7 |
| 4  89 1   | 5 7 2 | 89  3  6 |
| 67 58 67  | 4 3 9 | 2   58 1 |
+-----------+-------+----------+

Play this puzzle online at the Daily Sudoku site

For the sake of completeness (assuming I've missed nothing!) this is what I got after basics. I used Marty's 598 XY on r1c2,
though I couln't see how I could do it with the 895 XY pivoted in r1c8.

(Lot of v early birds today..............!)
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Marty R.



Joined: 12 Feb 2006
Posts: 5165
Location: Rochester, NY, USA

PostPosted: Thu Feb 02, 2012 3:44 pm    Post subject: Re: Feb 2 VH Reply with quote

hughwill wrote:
Code:

+-----------+-------+----------+
| 1  59 2   | 7 6 3 | 589 89 4 |
| 8  6  3   | 9 4 5 | 1   7  2 |
| 57 4  579 | 8 2 1 | 59  6  3 |
+-----------+-------+----------+
| 56 7  56  | 1 9 4 | 3   2  8 |
| 3  1  8   | 2 5 6 | 7   4  9 |
| 9  2  4   | 3 8 7 | 6   1  5 |
+-----------+-------+----------+
| 2  3  59  | 6 1 8 | 4   59 7 |
| 4  89 1   | 5 7 2 | 89  3  6 |
| 67 58 67  | 4 3 9 | 2   58 1 |
+-----------+-------+----------+

Play this puzzle online at the Daily Sudoku site

For the sake of completeness (assuming I've missed nothing!) this is what I got after basics. I used Marty's 598 XY on r1c2,
though I couln't see how I could do it with the 895 XY pivoted in r1c8.

(Lot of v early birds today..............!)

With that 895 in R1c8, doesn't removing the 5 from r9c2 do it?
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Luke451



Joined: 20 Apr 2008
Posts: 310
Location: Southern Northern California

PostPosted: Fri Feb 03, 2012 4:20 am    Post subject: Reply with quote

Hi, TerenceF. This is your grid as posted above. I thought you might be interested in a deadly pattern here that's fun and very easy to spot.
Code:
+------------+-------+-----------+
| 1   59 2   | 7 6 3 | 589 589 4 |
| 8   6  3   | 9 4 5 | 1   7   2 |
|*57  4 *57+9| 8 2 1 | 59  6   3 |
+------------+-------+-----------+
|*56  7 *56  | 1 9 4 | 3   2   8 |
| 3   1  8   | 2 5 6 | 7   4   9 |
| 9   2  4   | 3 8 7 | 6   1   5 |
+------------+-------+-----------+
| 2   3  59  | 6 1 8 | 4   59  7 |
| 4   89 1   | 5 7 2 | 89  3   6 |
|*567 58 *567| 4 3 9 | 2   58  1 |
+------------+-------+-----------+

The 567 pattern forms a MUG (Multivalue Universal Grave.) It's illegal (deadly) in that if allowed to exist the puzzle would have more than one solution.

Only the (9)r3c3 prevents the pattern, so it must be true.


Later on, hughwill posted this after basics grid.
Code:
+-----------+-------+----------+
| 1  59 2   | 7 6 3 | 589 89 4 |
| 8  6  3   | 9 4 5 | 1   7  2 |
|*57 4 *57+9| 8 2 1 | 59  6  3 |
+-----------+-------+----------+
|*56 7 *56  | 1 9 4 | 3   2  8 |
| 3  1  8   | 2 5 6 | 7   4  9 |
| 9  2  4   | 3 8 7 | 6   1  5 |
+-----------+-------+----------+
| 2  3  59  | 6 1 8 | 4   59 7 |
| 4  89 1   | 5 7 2 | 89  3  6 |
|*67 58 *67 | 4 3 9 | 2   58 1 |
+-----------+-------+----------+

Same concept, this is also a deadly pattern. Since all the cells in the pattern have only two candidates, it's a BUG-Lite (Bivalue Universal Grave-Lite.)

As before, only the (9)r3c3 prevents the pattern, so it must be true, puzzle solved.
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Orkamat



Joined: 20 Dec 2011
Posts: 10
Location: Winnipeg, Canada

PostPosted: Fri Feb 03, 2012 5:41 am    Post subject: Reply with quote

Hmmmm... I couldn't follow the skyscraper thingy, and I kept getting stuck on the "obvious" (and useless) XY-wings, but I did solve the puzzle with a simple coloring on 9 that eliminated the 9s from r1c8 and r3c7 (also r8c7, r7c3 and r1c2 for a little puzzle-solving overkill).

Where would I find a good description of the skyscraper?
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keith



Joined: 19 Sep 2005
Posts: 3179
Location: near Detroit, Michigan, USA

PostPosted: Fri Feb 03, 2012 7:37 am    Post subject: Reply with quote

Orkamat wrote:


Where would I find a good description of the skyscraper?


http://forum.enjoysudoku.com/strong-links-for-beginners-t3326.html

Keith
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Orkamat



Joined: 20 Dec 2011
Posts: 10
Location: Winnipeg, Canada

PostPosted: Fri Feb 03, 2012 9:14 am    Post subject: Reply with quote

Thanks for the skyscraper link. How peculiar that a strategy so closely related to others that I use regularly wasn't immediately obvious!

Also, the 9s in rows 1 & 8 form a finned x-wing (do they not?) eliminating the 9 from r3c7.
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Marty R.



Joined: 12 Feb 2006
Posts: 5165
Location: Rochester, NY, USA

PostPosted: Fri Feb 03, 2012 4:35 pm    Post subject: Reply with quote

Quote:
Also, the 9s in rows 1 & 8 form a finned x-wing (do they not?) eliminating the 9 from r3c7.

Correct.
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