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Mogulmeister
Joined: 03 May 2007 Posts: 1136

Posted: Sat Nov 20, 2021 8:12 am Post subject: VH Nov 20th 


Code: 
++++
 9 5 .  . 2 .  . 6 . 
 . . .  7 . .  5 4 . 
 . . .  . . .  . 3 9 
++++
 . . .  3 9 .  1 . . 
 . 9 .  1 . 6  . 5 . 
 . . 4  . 7 5  . . . 
++++
 8 4 .  . . .  . . . 
 . 1 7  . . 8  . . . 
 . 6 .  . 3 .  . 1 8 
++++



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Mogulmeister
Joined: 03 May 2007 Posts: 1136

Posted: Sat Nov 20, 2021 8:17 am Post subject: 


Code:  ++++
 9 5 18  4 2 3  78 6 17 
 236 238 12368  7 68 9  5 4 12 
 4 7 268  568 568 1  28 3 9 
++++
 56 28 56  3 9 24  1 278 247 
 7 9 238  1 48 6  234 5 234 
 1 238 4  28 7 5  2369 289 236 
++++
 8 4 2359  2569 1 27  23679 279 23567 
 235 1 7  2569 456 8  23469 29 23456 
 25 6 259  259 3 47  47 1 8 
++++ 
[corrected  thanks Tom]
Quote:  248 XY Wing in Boxes 4&5 so r5c3 < > 8
Other solutions are available.....

Last edited by Mogulmeister on Sat Nov 20, 2021 9:03 am; edited 2 times in total 

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TomC
Joined: 30 Oct 2020 Posts: 227 Location: Wales

Posted: Sat Nov 20, 2021 8:48 am Post subject: 


r3c7 <> 8 as then r1c3=8 and r2c5=8 leaving no <8> in row 5
Last edited by TomC on Sat Nov 20, 2021 9:21 am; edited 1 time in total 

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Mogulmeister
Joined: 03 May 2007 Posts: 1136

Posted: Sat Nov 20, 2021 9:25 am Post subject: 


A nice ALS
Set A is Yellow {2,4,8}
Set B is Purple {2,3,4,8}
Restricted common is 8
Common candidate 4 is eliminated from r5c5. 

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ZeroAssoluto
Joined: 05 Feb 2017 Posts: 685 Location: Rimini, Italy

Posted: Sat Nov 20, 2021 10:52 am Post subject: 


Hi everyone,
WWing 2,8 in r4c2,r6c4 SL with number 8 in r5c35 and 2 in r4c6,r6c2
or
Finned XWing with number 8 in r25c35 finned in r2c2 and 8 in r13c3
or
Finned Swordfish with number 8 in r135c357 finned in r3c4 and 8 in r2c5
Ciao Gianni 

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TomC
Joined: 30 Oct 2020 Posts: 227 Location: Wales

Posted: Sat Nov 20, 2021 11:04 am Post subject: 


Nice solutions all
Gianni, what would you describe my solution above as? 

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ZeroAssoluto
Joined: 05 Feb 2017 Posts: 685 Location: Rimini, Italy

Posted: Sat Nov 20, 2021 8:00 pm Post subject: 


TomC wrote:  Nice solutions all
Gianni, what would you describe my solution above as? 
I don't know.
What is the reasoning behind this choice? 

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TomC
Joined: 30 Oct 2020 Posts: 227 Location: Wales

Posted: Sat Nov 20, 2021 8:47 pm Post subject: 


Well Gianni, when I look for a solution one way I do it is to look at the whole of the puzzle to find patterns to help me solve.
Sometimes you need several of these patterns to combine to find the solution (like clues to solve a crime)
As it happens <8> was the clue to the pattern, as it was in your three solutions
Firstly, row 5 only contains two <8>'s in c3 and c5
Secondly, if you look at r3c7, if it is <8> then follow it left x2 up x1 and you get r2c5=8, follow it again left x2 up x1 and you get r1c3=8 (like a little <8> snake)
Thirdly, when these <8>'s drop down they knock out the <8>'s in row 5 (maybe like a skyscraper)
So r3c7 can't be an <8>
I've been doing sudoku for such a long time that this seems to be the best way to describe it to you
You could maybe say that if I see a pattern which will solve (that is no 8's in row 5) then I look for another pattern which will force the situation
Hope this helps
Tom 

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TomC
Joined: 30 Oct 2020 Posts: 227 Location: Wales

Posted: Sat Nov 20, 2021 8:50 pm Post subject: 


This method I used in the recent VH 111821 in other puzzles 

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Mogulmeister
Joined: 03 May 2007 Posts: 1136

Posted: Sat Nov 20, 2021 11:30 pm Post subject: 


Your method Tom is essentially contradiction based. The art of the impossible.Proof by negative assertion.
By plugging in a value you propagate changes in the fabric of a puzzle that might breakdown into impossibilities or contradictions. If your mind works that way the contradictions can be quickly seen. With me it is ALS. Both techniques mean looking at the structure of the puzzle first rather than applying a pattern.
This can be multichained although you only really need to have one of them to get your answer. This is how dongrave's long contradiction chains work. Very powerful because if your start point is a bivalue (2 digits in square) and you prove one leads to contradiction then the correct value is the other one on the square. Make it into a loop and the contradictions can be sometimes found at the start.
This is called a discontinuous nice loop.
Let's start one off with the above puzzle by sticking 8 in r3c7.
(2=8) r3c7(8=7)r1c7(7=1)r1c9(1=8)r1c3r5c3=r5c5(8)r23c5=r3c4(8)r3c7
Plug 8 into r3c7 and the loop creates a contradiction which you can see in two ways:
a)The loop puts an 8 in r3c4 and there is also an 8 in r3c7  Contradiction can't have two 8s in a row.
b)If the loop is followed all the way round it removes the 8 at the starting point of the loop so it must be false. The 8 removes itself.
However you view it r3c7 < > 8
There are at least 6 different types of contradictions (sometimes called logical absurdities)
1)Put too many of the same numbers into a house (row, column or box)
2)Remove a number totally from a house
3)Create a loop where the starting number is removed
4)Create a loop where a number is forced in even when removed at the start
5)Create multiple numbers in the same square location as the answer
6)Remove all numbers so the a square is vacant
Last edited by Mogulmeister on Sat Nov 20, 2021 11:57 pm; edited 1 time in total 

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Mogulmeister
Joined: 03 May 2007 Posts: 1136

Posted: Sat Nov 20, 2021 11:54 pm Post subject: 


Another wrinkle on this is one where you have a start cell with 2 or 3 candidates in it and a target cell which always reduces to the same value whatever candidate you choose to start with.
In this case you can do nothing about the start cell but you do now know the value of the target cell. 

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TomC
Joined: 30 Oct 2020 Posts: 227 Location: Wales

Posted: Sun Nov 21, 2021 9:09 am Post subject: 


You know my methods, Mogulmeister.
When you have eliminated the impossible, whatever remains, however improbable, must be the truth? 

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Mogulmeister
Joined: 03 May 2007 Posts: 1136

Posted: Sun Nov 21, 2021 4:32 pm Post subject: 


Always. 

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