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An Easy 17-clue Puzzle

 
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Sat Aug 11, 2007 9:25 pm    Post subject: An Easy 17-clue Puzzle Reply with quote

By Sam's definition, this one is very easy:
Code:
0 8 0  0 0 0  0 0 9 
0 1 2  0 0 0  0 0 0 
0 0 0  6 0 0  3 0 0 

0 7 0  0 0 0  2 8 0 
9 0 0  4 0 0  0 0 0 
0 0 0  3 0 0  0 0 0 

3 0 0  0 0 0  4 0 0 
5 0 0  0 8 0  0 0 0 
0 0 0  0 1 0  0 0 0
Keith
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Marty R.



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

PostPosted: Sun Aug 12, 2007 11:52 pm    Post subject: Reply with quote

Quote:
By Sam's definition, this one is very easy:


I don't know what factors Sam uses in assigning a grade. But I have seen puzzles from various sources graded Easy, Medium, Hard, or whatever, which require nothing more than subsets and locked candidates. That's all that's required with this one, but I'd consider it harder than some others because of the small number of givens and, thus, the greater number of pencil marks required. With all those candidates, it's easier to make a mechanical error.
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Asellus



Joined: 05 Jun 2007
Posts: 865
Location: Sonoma County, CA, USA

PostPosted: Mon Aug 13, 2007 8:03 pm    Post subject: Reply with quote

Marty R. wrote:
I'd consider it harder than some others because of the small number of givens and, thus, the greater number of pencil marks required.


On the other hand...

I don't start with pencil marks. Out of laziness, I first scan puzzles for obvious placements. For instance, it is obvious that R1C3 and then R5C2 are <3>. It is almost as obvious that R2C4 and R3C9 are <8>. The number of cells requiring pencil marks are reduced already: so much unnecessary pencil mark exertion avoided! However, the obvious placements are not done yet.

How far one can get with such placements will vary for different folks. But practicing this approach increases what becomes "obvious." In the case of this puzzle, I found it fairly easy to complete the entire thing in a few minutes without making a single pencil mark. In my book, that ranks it as Easy indeed.
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Marty R.



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

PostPosted: Mon Aug 13, 2007 9:12 pm    Post subject: Reply with quote

I just check each band and stack to see if a number is placeable in the 3rd box when it's already present in two boxes. After that I start right in with pencil marks. I could do more checking for obvious placements and maybe will try. But right now I don't know the tradeoff between fewer pencil marks versus extra time looking for placements.
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keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Mon Aug 13, 2007 10:00 pm    Post subject: Reply with quote

Actually, all this one requires is singles. "Very Easy", except the paucity of initial clues makes it harder, I think.

Keith
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Steve R



Joined: 24 Oct 2005
Posts: 289
Location: Birmingham, England

PostPosted: Mon Aug 13, 2007 11:51 pm    Post subject: Reply with quote

Maybe. At the same time, while there is no general connexion between difficulty and the number of clues, I don’t think this is true for minimal puzzles.

The huge majority of the minimal 17s are very easy. The average Sudoku Explainer rating is probably about 1. By contrast the minimal 38s have an average rating of 7.5.

Admittedly only 952 minimal 38s are known at present – Håvard Graff had a spell with his supercomputer and found another 951 – but it seems plausible that, when a puzzle requires as many as 38 clues to offer a unique solution, the clues must have a tendency not to interact very strongly.

Of course, “not to interact very strongly” is waiting for a precise definition….

Steve
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