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Friday Oct 28 Very Hard

 
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AZ Matt



Joined: 03 Nov 2005
Posts: 63
Location: Hiding under my desk in Phoenix AZ USA

PostPosted: Thu Nov 03, 2005 5:42 pm    Post subject: Friday Oct 28 Very Hard Reply with quote

Okay, maybe I was too cryptic with my earlier question on this puzzle; I didn't want to spoil it for others. I'll try again (the puzzle's been up long enough):

I used the following information to solve last Friday's puzzle. I am curious if this is how others solved the puzzle (i.e., is this "the solve," or did I miss an easier route), and how an expert would describe the solve:

R4= ___ (36) ___ ___ ___ (34)
R5= ___ ___ ___ ___ ___ (37)
R6= (78}(67) ___ ___ ___ (47)

I can see that r6c1 must be 8 because if r6c1 were 7, r6c2 would also have to be 7.

Is this just an x-y wing, a variation on forcing chains, or something else?
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David Bryant



Joined: 29 Jul 2005
Posts: 559
Location: Denver, Colorado

PostPosted: Thu Nov 03, 2005 6:55 pm    Post subject: The situation in box 4 Reply with quote

Oh, hi, AZ Matt. Glad to see you've joined the forum!

Sorry I didn't respond a little sooner. I saw your earlier post and thought it was interesting, but I had to reconstruct the position you had reached to answer, and just didn't get around to it for a little while.

I used the same cells at a critical point in this puzzle, but didn't use quite the same logic you did. Your method is pretty slick -- you got the right number at r6c1 without doing as much work as I had to.

Here's what I had when I got to the point you've illustrated.
Code:
 7/9   .    8    4   2/7   6    3    5    .
  .    .    6   3/7   .    1   2/9   8    4
  4    .    3    9    .    8   1/2  2/7   6

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

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

What I noticed was the triplet {3, 6, 7} in r4c2, r5c1, & r6c2. You noticed the contradiction implied by "7" in r6c1 without having yet uncovered the {3, 7} pair in row 5. Good work! dcb

PS As to a name for the method you used to place the "8" at r6c1, I'd just call it "forcing chains" and let it go at that.
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AZ Matt



Joined: 03 Nov 2005
Posts: 63
Location: Hiding under my desk in Phoenix AZ USA

PostPosted: Thu Nov 03, 2005 8:43 pm    Post subject: Thanks Reply with quote

Thanks David.

In the future I wll keep track of exactly where I am when I make what I think is a fairly good solve. I see from where you were at, the puzzle falls down with that solve (or is already solved another way), whereas after getting the eight, I still had some grinding to do to finish the puzzle. I suspect you get much farther along in a puzzle than I do before you get "stuck" and start looking for the "creative" solve.

I suppose someone could argue that what I did was "trial and error," since I didn't actually "see" the whole of the contradiction; what I saw was a group of pairs with an anomaluos 8, and just thought to look at it for awhile... oops, a discussion for another forum.
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alanr555



Joined: 01 Aug 2005
Posts: 194
Location: Bideford Devon EX39

PostPosted: Sat Nov 05, 2005 1:02 am    Post subject: Re: The situation in box 4 Reply with quote

Code:

> Here's what I had when I got to the point you've illustrated.
 7/9   .    8    4   2/7   6    3    5    .
  .    .    6   3/7   .    1   2/9   8    4
  4    .    3    9    .    8   1/2  2/7   6

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

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

> What I noticed was the triplet {3, 6, 7} in r4c2, r5c1, & r6c2. You
> noticed the contradiction implied by "7" in r6c1 without having yet
> uncovered the {3, 7} pair in row 5. Good work! 

> PS As to a name for the method you used to place the "8" at r6c1, I'd
> just call it "forcing chains" and let it go at that.

+++
Surely the setting of r6c1 to '8' is really an example of
"Only place in the region for specified digit"?

The region is r4c1 to r6c3 and no candidate profile for any cell in that
region contains an '8' except r6c1. However, I agree that the 'triple'
leading to exclusion of the 7 leads to the same result.

Alan Rayner BS23 2QT
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