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hughwill
Joined: 05 Apr 2010
Posts: 424
Location: Birmingham UK
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 Posted: Sun Aug 09, 2015 11:13 am Post subject: Aug 9 VH |
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After basics: | Code: |
+------------+-----------+---------+
| 18 178 68 | 57 457 29 | 29 3 46 |
| 5 23 4 | 39 8 6 | 29 7 1 |
| 9 237 236 | 1 47 23 | 8 5 46 |
+------------+-----------+---------+
| 4 6 5 | 8 9 7 | 1 2 3 |
| 2 9 1 | 6 3 5 | 4 8 7 |
| 7 38 38 | 2 1 4 | 6 9 5 |
+------------+-----------+---------+
| 38 4 89 | 39 57 1 | 57 6 2 |
| 6 5 7 | 4 2 8 | 3 1 9 |
| 13 12 29 | 57 6 39 | 57 4 8 |
+------------+-----------+---------+
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Play this puzzle online at the Daily Sudoku site
Took me a very long time to see the simple | Quote: | | 392 XY-wing pivot r9c6 |
which solves it.
Hugh
(fra Stavanger i dag) |
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bat999
Joined: 09 Jul 2015
Posts: 55
Location: UK
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| Posted: Sun Aug 09, 2015 12:31 pm Post subject: |
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| Code: | .------------------.--------------.------------.
| 18 1278 268 | 57 457 29 | 289 3 46 |
| 5 a23 4 | b39 8 6 | 29 7 1 |
| 9 2378 2368 | 1 47 23 | 28 5 46 |
:------------------+--------------+------------:
| 4 6 5 | 8 9 7 | 1 2 3 |
| 2 9 1 | 6 3 5 | 4 8 7 |
| 7 38 38 | 2 1 4 | 6 9 5 |
:------------------+--------------+------------:
| 38 4 d89 | c39 57 1 | 57 6 2 |
| 6 5 7 | 4 2 8 | 3 1 9 |
| 13 1-2 e29 | 57 6 39 | 57 4 8 |
'------------------'--------------'------------' |
(2=3)r2c2 - (3=9)r2c4 - (9)r7c4 = r7c3 - (9=2)r9c3 => -2 r9c2; stte
Last edited by bat999 on Mon Aug 10, 2015 8:50 pm; edited 1 time in total |
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dongrave
Joined: 06 Mar 2014
Posts: 568
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| Posted: Sun Aug 09, 2015 3:06 pm Post subject: |
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| Hey! Wait a minute! Hugh's starting grid looks to me like he already applied the two 39-2 XY-wings with pivot at r2c4 to eliminate the 2 from r1c23 and the 2 from r3c7. Did I miss some basic moves that eliminate them? |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Sun Aug 09, 2015 3:29 pm Post subject: |
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| dongrave wrote: | | Hey! Wait a minute! Hugh's starting grid looks to me like he already applied the two 39-2 XY-wings with pivot at r2c4 to eliminate the 2 from r1c23 and the 2 from r3c7. Did I miss some basic moves that eliminate them? |
I saw the grid and thought I screwed up the basics since I had 28 in r3c7 and found that the wing pivoted in r2c4 wasn't enough. |
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dongrave
Joined: 06 Mar 2014
Posts: 568
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| Posted: Sun Aug 09, 2015 7:33 pm Post subject: |
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| I went back and looked at it again and it looks to me like Bat's grid is right after basics and then in order to solve it using just standard VH moves (i.e. wings), you need to apply at least one of the two 39-2 XY-wings with pivot at r2c4 (either one will do) along with Hugh's XY-wing. Someone let me know if I'm wrong. |
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hughwill
Joined: 05 Apr 2010
Posts: 424
Location: Birmingham UK
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| Posted: Mon Aug 10, 2015 9:46 pm Post subject: |
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Yes, mea culpa. My 'basics' must have included a dreaded 'correct mistake'
leading to an unearned conclusion. |
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TerenceF
Joined: 21 Dec 2007
Posts: 26
Location: Takapuna, NZ
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| Posted: Sun Aug 16, 2015 11:46 am Post subject: |
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Is this an allowable step?
Starting from Hugh's grid, if r3c5 is 4, then we have a chain of cells with 57 as the candidates, viz r1c4,r1c5,r7c5,r7c7,r9c7,r9c4,r1c4 (again) so the puzzle would have no unique solution.
So r3c5 <> 4, after which it reverts to singles. |
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bat999
Joined: 09 Jul 2015
Posts: 55
Location: UK
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| Posted: Sun Aug 16, 2015 2:02 pm Post subject: |
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| TerenceF wrote: | Is this an allowable step?
... so the puzzle would have no unique solution.... |
Hi
There are a bunch of uniqueness tests.
Some of them are quite complicated. 
Here, for example --> http://sudopedia.enjoysudoku.com/Solving_Technique.html#Uniqueness
You need to show that your "allowable step" would cause it to fail a uniqueness test.  |
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dongrave
Joined: 06 Mar 2014
Posts: 568
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| Posted: Sun Aug 16, 2015 5:52 pm Post subject: |
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| TerenceF wrote: | Is this an allowable step?
Starting from Hugh's grid, if r3c5 is 4, then we have a chain of cells with 57 as the candidates, viz r1c4,r1c5,r7c5,r7c7,r9c7,r9c4,r1c4 (again) so the puzzle would have no unique solution.
So r3c5 <> 4, after which it reverts to singles. |
Any of the experts out there like Marty or Keith could tell you at a glance! I remember hearing about 'deadly patterns' from them a while ago and your list of 57 cells looks to me like one of those well-known 6-cell deadly patterns - which would mean that the answer to your question is - yes, your solution is legit. |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Sun Aug 16, 2015 7:32 pm Post subject: |
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| Don's opinion notwithstanding, I am not an expert on anything. There have been a few people, George Woods' name comes to mind, who eschew uniqueness-based solutions. They never said why and I can't understand why they feel as they do, but the vast majority accepts those solutions. |
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