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can this be done without resorting to brute force?

 
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tdushane



Joined: 26 Feb 2006
Posts: 1

PostPosted: Sun Feb 26, 2006 11:16 am    Post subject: can this be done without resorting to brute force? Reply with quote

The puzzle appeared, in all places, in USA Today on Jan 27, 2006:

X X X . . X X 1 . . 2 X X
X 3 X . . X X 4 . . 5 X X
X 6 2 . . X X 7. . X X X

5 X X . . X 6 X . . X X 7
2 X X . . X 4 X . . X X 8
9 X X . . X 3 X . . X X 1

X X X . . 4 X X . . 1 9 X
X X 8 . . 5 X X . . X 6 X
X X 4 . . 2 X X . . X X X

You fairly quickly get to a situation where each row, column and 3x3 block of 9 has a pattern with candidates, say, 24 in two of them, 357 (or some combo of these) in three of them.

Thanks for telling me if I'm missing something obvious.

P.S.By brute force, I mean taking the puzzle, at a certain stage, and using, for example, one of the situations where the candidates might be 2 or 3 in a square, and just trying each possibiity to see which leads to a contradiction, and which does not (which gives me that particular square). I had to do this in three layers on this one, the most I've ever had to do.
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keith



Joined: 19 Sep 2005
Posts: 3136
Location: near Detroit, Michigan, USA

PostPosted: Sun Feb 26, 2006 12:45 pm    Post subject: USA Today puzzle Reply with quote

This puzzle was discussed in this forum. Here is the link:

http://www.dailysudoku.co.uk/sudoku/forums/viewtopic.php?t=422

Keith
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Marty R.



Joined: 12 Feb 2006
Posts: 5063
Location: Rochester, NY, USA

PostPosted: Mon Feb 27, 2006 10:26 pm    Post subject: Reply with quote

Keith, that thread you linked to has a couple of extra clues in it. Specifically, the "2" and "5" in box 2. No wonder I got backed into a corner right off the bat, but I should have noticed the asymetry.

From the linked thread:

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



Joined: 19 Sep 2005
Posts: 3136
Location: near Detroit, Michigan, USA

PostPosted: Mon Feb 27, 2006 11:15 pm    Post subject: Original puzzle Reply with quote

Marty,

The original puzzle is posted in the USA Today archive:

http://puzzles.usatoday.com/sudoku/archive/2006/01/27/

The asymmetric values you noticed in block two are the only two cells the person was able to solve. They are not in the original.

Keith
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Marty R.



Joined: 12 Feb 2006
Posts: 5063
Location: Rochester, NY, USA

PostPosted: Tue Feb 28, 2006 5:38 pm    Post subject: Reply with quote

Quote:
The asymmetric values you noticed in block two are the only two cells the person was able to solve. They are not in the original.

Oops. Guess I wasn't paying enough attention. Thanks.
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