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last week's extreme #60

 
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storm_norm



Joined: 18 Oct 2007
Posts: 1741

PostPosted: Tue Nov 20, 2007 6:09 pm    Post subject: last week's extreme #60 Reply with quote

this is from sudoku.org.uk, which has changed its address.
basics get to here:

Code:
------------------------------------------------------------------------------|
| 8       4       9       | 53      2       53      | 1       67      67      |
| 5       6       2       | 1       7       9       | 3       4       8       |
| 1       7       3       | 468     48      468     | 5       9       2       |
------------------------------------------------------------------------------|
| 9       238     16      | 3468    1348    7       | 2468    1268    5       |
| 367     2358    1567    | 34568   13458   34568   | 9       12678   1467    |
| 4       58      1567    | 9       158     2       | 68      1678    3       |
------------------------------------------------------------------------------|
| 2       1       4567    | 34578   3458    3458    | 468     368     9       |
| 37      9       47      | 23478   6       1348    | 248     5       14      |
| 36      35      8       | 2345    9       1345    | 7       1236    146     |
------------------------------------------------------------------------------|


norm
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keith



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

PostPosted: Tue Nov 20, 2007 9:16 pm    Post subject: Reply with quote

There is a chain in R89 that eliminates <5> in R9C6. Then, no more chains. (According to Sudoku Susser.)

Keith
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ravel



Joined: 21 Apr 2006
Posts: 536

PostPosted: Wed Nov 21, 2007 8:22 am    Post subject: Reply with quote

Code:
------------------------------------------------------------------------------|
| 8       4       9       | 53      2       53      | 1      C67     C67      |
| 5       6       2       | 1       7       9       | 3       4       8       |
| 1       7       3       | 468     48      468     | 5       9       2       |
------------------------------------------------------------------------------|
| 9       238     16      | 3468    1348    7       | 2468    1268    5       |
|C367     2358    1567    | 34568   13458   34568   | 9       12678  C1467    |
| 4      B58      1567    | 9       158     2       |B68      1678    3       |
------------------------------------------------------------------------------|
| 2       1      A4567    | 34578   3458    3458    |B468    C368     9       |
|-37      9       47      | 23478   6       1348    | 248     5       14      |
|A36     B35      8       | 2345    9       1345    | 7       1236    146     |
------------------------------------------------------------------------------|
This seems to be an awkward puzzle, that does not allow an elegant solution (at least i cant see one). You have the choice between a couple of shorter chains or a long and complicated one like this (indeed a triple forcing chain):
r8c1=3 => r9c1=6 => r7c3<>6
r8c1=3 => r9c2=5 => r6c2=8 => r6c7=6 => r7c7<>6
r8c1=3 => r5c1=7 => r5c9<>7 => r1c9=7 => r1c8=6 => r7c8<>6
So all 6's in row 7 are killed, therefore r8c1 cannot be 3.
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