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UR Help - BB 5/20 Super Hard

 
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Kdelle



Joined: 20 Mar 2008
Posts: 59
Location: Hudson, NH
PostPosted: Tue May 27, 2008 11:48 pm    Post subject: UR Help - BB 5/20 Super Hard Reply with quote
Code:

+--------------+-----------------+-------------+
| 7   146  469 | 249   5     249 | 8  16  3    |
| 456 3456 2   | 48    1     34  | 9  567 67   |
| 15  8    39  | 6     39    7   | 25 4   12   |
+--------------+-----------------+-------------+
| 29  45   1   | 3     2479  8   | 6  257 2479 |
| 56  7    36  | 249   249   1   | 35 8   249  |
| 29  34   8   | 5     2479  6   | 1  237 2479 |
+--------------+-----------------+-------------+
| 468 2    467 | 1     3478  5   | 37 9   68   |
| 18  9    5   | 278   6     23  | 4  237 18   |
| 3   146  467 | 24789 24789 249 | 27 16  5    |
+--------------+-----------------+-------------+

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After some basics this is where I am. I've read through th BB proof, and I am confused.

My thinking was that R1C28 and R9C28 formed a UR only if one of R1C2 or R9C2 was a 4. But that would leave Block 4 with no 4.

BB proof says that Block 4 must have a 4, eliminating the 4 in R1C2 and R9C2, leaving 16 in what I had thought was a "deadly pattern". Could someone explain how my logic is off? Do I need to revisit the whole UR discussion? I really thought I understood it. Thanks in advance!!!
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Marty R.



Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
PostPosted: Wed May 28, 2008 12:34 am    Post subject: Reply with quote
Quote:
Could someone explain how my logic is off?

Yes. UR theory, or logic, or whatever it's called, is based on the fact that the four corners of the rectangle must be in two boxes. In other words, the entire UR must be within one band or stack. Your example has four boxes so no deadly pattern develops under that circumstance.

Here's a primer on URs if you're interested.

http://www.sudoku.com/boards/viewtopic.php?p=29105#29105
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Kdelle



Joined: 20 Mar 2008
Posts: 59
Location: Hudson, NH
PostPosted: Wed May 28, 2008 12:46 am    Post subject: Reply with quote
Marty R. wrote:
Quote:
Could someone explain how my logic is off?

Yes. UR theory, or logic, or whatever it's called, is based on the fact that the four corners of the rectangle must be in two boxes. In other words, the entire UR must be within one band or stack. Your example has four boxes so no deadly pattern develops under that circumstance.

Here's a primer on URs if you're interested.

http://www.sudoku.com/boards/viewtopic.php?p=29105#29105


Thanks. I'll reread the thread....
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storm_norm



Joined: 18 Oct 2007
Posts: 1741
PostPosted: Wed May 28, 2008 6:04 am    Post subject: Reply with quote


if anyone was wondering, there is a w-wing that solves it marked in the image above. the {3,4} pair along with the strong links on 4 in col 1 eliminates the 3 in r8c6...

(3=4)r2c6-(4)r2c1=(4)r7c1-(4=3)r7c5; r8c6 <> 3 Shocked
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