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Stuck !

 
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Earl



Joined: 30 May 2007
Posts: 643
Location: Victoria, KS

PostPosted: Wed Aug 22, 2007 3:55 pm    Post subject: Stuck ! Reply with quote

Since the Aug 22 VH was solved too quickly, I went looking for a challenge and promptly got my self stuck. Help!

Code:

+-------------------+------------+------------------+
| 25   4     2357   | 1  26  69  | 35789 5789   578 |
| 18   9     18     | 7  5   3   | 4     2      6   |
| 256  2367  23567  | 29 4   8   | 3579  579    1   |
+-------------------+------------+------------------+
| 9    23678 123678 | 28 126 5   | 178   4      78  |
| 4    678   1678   | 89 3   679 | 15789 156789 2   |
| 1268 5     12678  | 4  126 679 | 1789  16789  3   |
+-------------------+------------+------------------+
| 3    28    258    | 6  9   4   | 12578 1578   578 |
| 7    1     4      | 5  8   2   | 6     3      9   |
| 2568 268   9      | 3  7   1   | 258   58     4   |
+-------------------+------------+------------------+

Play this puzzle online at the Daily Sudoku site
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TKiel



Joined: 22 Feb 2006
Posts: 292
Location: Kalamazoo, MI

PostPosted: Wed Aug 22, 2007 4:12 pm    Post subject: Reply with quote

There's a strong link exclusion for r1c1.
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Johan



Joined: 25 Jun 2007
Posts: 206
Location: Bornem Belgium

PostPosted: Wed Aug 22, 2007 7:36 pm    Post subject: Reply with quote

Earl,

Take a look in C9 for Tracy's strong link exclusion.
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Asellus



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

PostPosted: Wed Aug 22, 2007 9:51 pm    Post subject: Reply with quote

You can make the same elimination with a simple finned fish, also involving C9. Plus, there is an XYZ-Wing in the lower right region of the grid.
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Earl



Joined: 30 May 2007
Posts: 643
Location: Victoria, KS

PostPosted: Thu Aug 23, 2007 12:29 am    Post subject: Stuck Reply with quote

I see the finned fish, a bit of a variation.
I see the xyz wing, but not that it eliminates anything.
I see the strong link in C9 and box 7, but I do not see how they are strongly linked, unless you can eliminate the 5's from R7C7,8.

Thanks for the help.

Earl
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Mogulmeister



Joined: 03 May 2007
Posts: 703

PostPosted: Thu Aug 23, 2007 2:10 am    Post subject: Reply with quote

Earl,

When 2 strong links intersect (share) a house, then if the ends (or pincers) can both see a candidate it can be eliminated. In this case the pincers are r1c9 and r9c1.

Look at the diagrams below.

The 2 strong links on 5 are in box 7 and column 9. They intersect on row 7. Lets say green is true and orange is false.



The only outcome that would cause a failure is if r9c1 and r1c9 were both false at the same time. However, as you can see - set either pincer to false and the other pincer is always true.



The 5 in r1c1 can't survive.
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Marty R.



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

PostPosted: Thu Aug 23, 2007 4:06 am    Post subject: Reply with quote

I'll take the simple Finned X-Wing in columns 3 and 9, since I'd have found that before the strong links, indeed, if I ever would have found them.
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Asellus



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

PostPosted: Thu Aug 23, 2007 7:00 am    Post subject: Reply with quote

This strong link type of elimination is another "Color Wing" example. R7 provides the weak link "Bridge". Some brief reflection shows that it is essentially the same as a Skyscraper or a Kite or Turbot Fish in its logical structure, which is why I find the more general notion of a "Color Wing" to be so useful.

All of these named "sub-types" often have the structure of two 2-cell strong links joined together by a weak link.

Examining a puzzle for the locations of strong links is often helpful in finding solutions, in my experience. They may lead to useful simple coloring or a simple "Color Wing" (aka multi-coloring). If the links are not extensive, then the coloring is not difficult to do in ones head. (After all, who needs to color their Skyscrapers and Kites?) That is a boon for pencil and paper types.

It seems a shame to be able to spot Skyscrapers but not be able to spot the logically identical and no more complex structure discussed above.

PS: While it is true that it if one of the "pincers" is false then the other must be true (and this fact alone is enough to justify the elimination), it is also true that it is possible for both "pincers" to be true. (To see this, suppose that R7C8 is the correct <5> cell.) This is a consequence of the weak link.
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Asellus



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

PostPosted: Thu Aug 23, 2007 7:05 am    Post subject: Reply with quote

Earl wrote:
I see the xyz wing, but not that it eliminates anything.

My mistake! It is a useless XYZ-Wing, alas.
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Mogulmeister



Joined: 03 May 2007
Posts: 703

PostPosted: Thu Aug 23, 2007 8:49 am    Post subject: Reply with quote

I think more formally this structure is known as an "X-cycle".

Quote:
to spot the logically identical and no more complex structure discussed above.



I absolutely agree - I think it is another evolutionary thing that gets taken on as your skills improve. Initially you work with strong links "end-to-end" until you start to notice how conjugates can be weakly linked.
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Hasel



Joined: 23 Aug 2007
Posts: 4

PostPosted: Thu Aug 23, 2007 9:46 am    Post subject: Reply with quote

I find it difficult ......





Hasel
http://classicalpearls.blog.com/
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Earl



Joined: 30 May 2007
Posts: 643
Location: Victoria, KS

PostPosted: Thu Aug 23, 2007 3:03 pm    Post subject: Stuck Reply with quote

Thanks to all of you for this discussion of strong pairs, weakly linked.
It is a new insight for me.

I have avoided "multiple coloring" since it seemed so complex - no fun. Strong pairs, weakly linked, I can handle.

Earl
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