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Oct 20 DB

 
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Earl



Joined: 30 May 2007
Posts: 677
Location: Victoria, KS

PostPosted: Sat Oct 20, 2007 12:37 pm    Post subject: Oct 20 DB Reply with quote

Here is the Oct 20 DB as in the St. Louis paper.

I did it with two xy-chains. Hope someone finds a neater solution.

Earl


Code:

+-------+-------+-------+
| . 2 . | 5 1 . | . . . |
| . 8 . | . 6 . | 1 . 2 |
| . . . | . . 8 | 3 6 . |
+-------+-------+-------+
| 2 . . | 3 . . | . 8 1 |
| . 9 5 | . . . | 6 2 . |
| 1 7 . | . . 6 | . . 4 |
+-------+-------+-------+
| . 3 2 | 9 . . | . . . |
| 4 . 9 | . 7 . | . 3 . |
| . . . | . 3 2 | . 7 . |
+-------+-------+-------+

Play this puzzle online at the Daily Sudoku site
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TKiel



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

PostPosted: Sat Oct 20, 2007 1:14 pm    Post subject: Reply with quote

Basics stop here:

Code:

 *--------------------------------------------------*
 | 369  2    67   | 5    1    379  | 89   4    78   |
 | 359  8    47   | 47   6    39   | 1    59   2    |
 | 59   45   1    | 247  29   8    | 3    6    57   |
 |----------------+----------------+----------------|
 | 2    46   46   | 3    59   59   | 7    8    1    |
 | 8    9    5    | 17   4    17   | 6    2    3    |
 | 1    7    3    | 28   28   6    | 59   59   4    |
 |----------------+----------------+----------------|
 | 7    3    2    | 9    58   4    | 58   1    6    |
 | 4    16   9    | 168  7    15   | 2    3    58   |
 | 56   156  8    | 16   3    2    | 4    7    9    |
 *--------------------------------------------------*


Seemingly worthless <156> XY-wing with pivot at r8c2. Coloring the pincers leads to the exclusion of <5> in r3c1 and solves the puzzle.
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Johan



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

PostPosted: Sat Oct 20, 2007 3:20 pm    Post subject: Reply with quote

A 7-cell XY-chain starting with <6> in R9C1, that ends with <5> in R3C1, which erases <5> in R2C1, was the final step for me.
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Marty R.



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

PostPosted: Sat Oct 20, 2007 3:24 pm    Post subject: Reply with quote

I eliminated the 9 from r1c1 to break up the deadly pattern, but that move affected nothing. But I then spotted the XY-Wing and made the identical move as Tracy did to finish it off. I just want to add that the coloring of pincers is a new technique to me within the last month or so and it has turned out to be a very powerful weapon.
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keith



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

PostPosted: Sun Oct 21, 2007 6:00 pm    Post subject: Reply with quote

Code:
+-------------+-------------+-------------+
| 369  2  67  | 5   1   379 | 89  4   78  |
| 35-9 8  47  | 47  6   39  | 1   59$ 2   |
| 59$ 45  1   | 247 29  8   | 3   6   57  |
+-------------+-------------+-------------+
| 2   46  46  | 3   59# 59  | 7   8   1   |
| 8   9   5   | 17  4   17  | 6   2   3   |
| 1   7   3   | 28  28  6   | 59# 59  4   |
+-------------+-------------+-------------+
| 7   3   2   | 9   58@ 4   | 58@ 1   6   |
| 4   16  9   | 168 7   15  | 2   3   58  |
| 56  156 8   | 16  3   2   | 4   7   9   |
+-------------+-------------+-------------+

Marty has pointed out the Unique X-wing on <39> in R12C16. You can eliminate <9> from R1C1.

There is also a UR on <16> in R89C24. With strong links on <1> and <6>, you can take out <1> from R8C4 and <6> from R9C2.

None of these eliminations are of any help.

There is a W-wing on <9>: With the strong link on <5> marked @, one or both of the cells marked # is <9>. Then, by coloring, one or both of the cells marked $ is <9>, and you can eliminate <9> as a candidate in R2C1.

The result is the same as that obtained by Tracy.

Keith
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dejsmith



Joined: 23 Oct 2005
Posts: 42

PostPosted: Tue Oct 23, 2007 7:11 pm    Post subject: Reply with quote

Marty, Tracy

As usual, I am a few days behind; but I seem to have been gone for the discussion on "coloring the pincers". Could you explain that or point me in the right direction to read about this technique? I gave up & settled on solving this puzzle with a contradiction off of the R89C24 UR.

Thanks...Dave
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Marty R.



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

PostPosted: Tue Oct 23, 2007 9:32 pm    Post subject: Reply with quote

Dave,

Let me try with an example.

Code:

+--------+--------+---------+
| .  . . | .  . . | . .   . |
| .  . . | .  . . | . .   . |
| .  . . | .  . . | . 345 . |
+--------+--------+---------+
| .  . . | .  . . | . .   . |
| .  . . | .  . . | . .   . |
| 34 . . | .  . . | . .   . |
+--------+--------+---------+
| .  . . | .  . . | . .   . |
| .  . . | .  . . | . .   . |
| 24 . . | 23 . . | . 368 . |
+--------+--------+---------+

Play this puzzle online at the Daily Sudoku site

Note the XY-Wing in the lower left, which has 3 as the pincer number. Assume the two trivalue cells are strong links on 3. So we create a three-cell chain on 3. If the pincer in r9c4 is a 3, then r3c8 must also be a 3 because of the strong links, so r3c8 also works as a pincer with r6c1 and creates further potential eliminations.

Hope that's clear enough.
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keith



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

PostPosted: Tue Oct 23, 2007 10:41 pm    Post subject: Reply with quote

Dave,

Take a look at this:

http://www.dailysudoku.com/sudoku/forums/viewtopic.php?t=2028

The result of a W-wing is the same as that of an XY-wing: One, or both of two pincer cells must be W (or Z). With coloring, you can extend the pincer cells to other cells that must have the same value.

I agree with Marty that this is a very powerful technique. We should probably rewrite the explanation given in the link above, now that our understanding has matured.

Best wishes,

Keith
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dejsmith



Joined: 23 Oct 2005
Posts: 42

PostPosted: Wed Oct 24, 2007 4:56 pm    Post subject: Reply with quote

Thanks guys

As usual, I have printed out the link because I am sure this will take me a while to absorb. Thanks for continuing your efforts to enlighten us all on Sam's site. Although I do not write often since I am always behind in my puzzle solving, I continue to read & enjoy learning from all of you.

Dave
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