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Marty R.
Joined: 12 Feb 2006 Posts: 5770 Location: Rochester, NY, USA

Posted: Wed Jul 15, 2015 5:46 pm Post subject: Dan's July14 puzzle. 


Dan (arkietech) originally posted this on the Enjoy Sudoku forum.
It's different and I thought some of you might find it interesting. It'll test one part of your game. There is a small variety of solutions. I did not check thoroughly, but I think there are no available standard VH moves.
Code: 
++++
 . 1 .  2 3 .  . . . 
 2 . 4  1 5 .  6 . . 
 . 7 .  . . .  . . . 
++++
 . . .  4 . .  . 1 . 
 8 6 .  . . .  5 . 9 
 . . .  . . .  3 . 7 
++++
 . . .  . 1 .  7 . . 
 . . .  . . .  . 8 . 
 . . .  6 7 9  1 . 3 
++++

Play this puzzle online at the Daily Sudoku site 

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Clement
Joined: 24 Apr 2006 Posts: 1011 Location: Dar es Salaam Tanzania

Posted: Fri Jul 17, 2015 4:44 pm Post subject: 


Code: 
++++
 69 1 689  2 3 e68  4 7 5 
 2 3 4  1 5 7  6 9 8 
 5 7 c68  9 d68 4  2 3 1 
++++
 379 5 379  4 69 36  8 1 2 
 8 6 a13  7 2 31  5 4 9 
 19 4 2  5 89 f18  3 6 7 
++++
 36 9 b36  8 1 2  7 5 4 
 17 2 17  3 4 5  9 8 6 
 4 8 5  6 7 9  1 2 3 
++++
 XY Chain
(1=3)r5c3(3=6)r7c3(6=8)r3c3(8=6)r3c5(6=8)r1c6(8=1)r6c6 => 1r5c6; stte 

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Marty R.
Joined: 12 Feb 2006 Posts: 5770 Location: Rochester, NY, USA

Posted: Fri Jul 17, 2015 7:32 pm Post subject: 


My first solution was the BUG+3. Then I looked at the 363969 DP, which gave the identical elimination.
I had some difficulty getting through the basics. 

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dongrave
Joined: 06 Mar 2014 Posts: 339

Posted: Sat Jul 18, 2015 2:29 am Post subject: 


I sure had a tough time with this one! How could I be left with so few unsolved cells (each with only 2 or 3 candidates) and not be able to find a standard VH move (i.e. xwing, xywing, or xyzwing)? I finally gave up. If there is one there, I sure don't see it.
Oh well, what about those forcing chains that I always see represented in Eureka notation? I'll go find one of those. Then it occurred to me  those chains are really nothing more than a simple proof by contradiction. Who cares what they call them or how they're represented? They're all the same thing! They assume that a cell is the wrong value and then arrive at a contradiction.
So I just took the first unsolved bivalue cell (r1c1 which is either 6 or 9) and assumed that it was a 6. Since it didn't lead to a contradiction, I assumed that it was the other value (i.e. 9) and I quickly saw that if r1c1=9, then then r1c3<>9 so r7c3=3 (because of the naked 68 pair in c3) so r5c3=1 so r6c1=9 but it can't be 9 because r1c1 is 9!
Wait a minute! Is it really that simple? I'm starting to think that it is! Am I wrong or are these 'forcing chains' nothing more than 'trial and error' proofs by contradiction? If they are, then I began wondering  since I could have begun with any of the unsolved cells, which solution would be the 'best'? The purist in me tells me that it would be the one with the fewest number of steps before arriving at a contradiction (but maybe that's just me). Anyway, I'd appreciate any thoughts about this. Thanks, Don. 

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bat999
Joined: 09 Jul 2015 Posts: 55 Location: UK

Posted: Sat Jul 18, 2015 4:36 am Post subject: 


dongrave wrote:  ... since I could have begun with any of the unsolved cells, which solution would be the 'best'?... 
Hi
When I asked this question at the enjoysudoku forum there was mention of a "b/b plot".
Here > http://forum.enjoysudoku.com/june142015t32504.html#p244578
I don't use the method though, it looks tedious.
I'm still using "trial and error methods" to find likely places to start.
Sometimes it leads to a verity or contradiction that kicks the can down the road.
Sometimes it leads to a verity or contradiction that solves the puzzle.
Sometimes it leads to a dry hole. 

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Marty R.
Joined: 12 Feb 2006 Posts: 5770 Location: Rochester, NY, USA

Posted: Sat Jul 18, 2015 4:05 pm Post subject: 


I took a look at that thread on Enjoy Sudoku and don't understand a thing I'm reading. Maybe that's why I have such difficulty finding chains. 

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dongrave
Joined: 06 Mar 2014 Posts: 339

Posted: Sun Jul 19, 2015 12:52 am Post subject: 


Hey! Thanks for the link batman! I appreciate it! I just clicked on it and took a quick glance and it looks like real interesting reading! If it didn't make sense to Marty, it'll probably have my head spinning  but I'm going to check it out anyway! Thanks again, Don. 

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bat999
Joined: 09 Jul 2015 Posts: 55 Location: UK

Posted: Sun Jul 19, 2015 5:09 pm Post subject: 


dongrave wrote:  ... and it looks like real interesting reading!.. 
Hi
As I said, I don't use that method, it looks tedious to me.
I am impatient.
By the time I had gone through that procedure I could have made several "trial and error" attempts, and probably have come up with a result.
If that b/b plot was widely used I would expect it to be mentioned occasionally, but no.
Google only throws up a 10 year old thread here > http://forum.enjoysudoku.com/niceloopsforadvancedlevelplayersbbplott2143.html
Maybe it was popular back in the day, but it seems to have died a death.
When I see people post super onestep solutions to puzzles I think "How the heck did he know to look there?"
There was no big red arrow saying "Look here first mate!".
It could be that the dirty little secret is we all use "trial and error" attempts to some degree.
Perhaps with experience we will get a better idea which are good places to try first when looking for chains/loops/contradictions/etc. 

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Marty R.
Joined: 12 Feb 2006 Posts: 5770 Location: Rochester, NY, USA

Posted: Sun Jul 19, 2015 6:09 pm Post subject: 


Maybe we all do indeed use trial and error. There are some very goodplayers doing these puzzles and some of them can see the grid in ways that I can't. I'm extremely impressed with some of the ingenious solutions. However, you can't dismiss the fact that some are using computer help to find moves. But even those who do use computers know a lot based on conversations with them. 

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