Help a sudoku newbie

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Help a sudoku newbie

Postby CreatureX » Fri Jun 16, 2006 6:02 pm

This "moderate" sudoku puzzle is from the June 15, 2006 edition of the Los Angeles Times. I'm a newbie to Sudoku so forgive me if this is too easy.

I've solved the puzzle to this point using "logic". However, I can't seem to continue the puzzle without having to "guess" at the next number. I thought you never had to guess in solving a Sudoku puzzle. Is this a bad Sudoku puzzle or am I doing something wrong? Thanks for the help.

|*|8|3| |5|*|1| |9|*|4|
|9|5|2| |6|8|4| |7|3|1|
|1|*|4| |9|3|*| |5|*|8|

|8|2|6| |7|4|5| |3|1|9|
|4|1|7| |8|9|3| |6|5|2|
|5|3|9| |2|1|6| |8|4|7|

|*|4|8| |1|5|9| |2|7|*|
|*|9|1| |3|6|*| |4|8|5|
|*|*|5| |4|*|8| |1|9|*|
CreatureX
 
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Postby Sped » Fri Jun 16, 2006 7:31 pm

Code: Select all
 
 *-----------------------------------------------------------*
 | 67    8     3     | 5     27    1     | 9     26    4     |
 | 9     5     2     | 6     8     4     | 7     3     1     |
 | 1     67    4     | 9     3     27    | 5     26    8     |
 |-------------------+-------------------+-------------------|
 | 8     2     6     | 7     4     5     | 3     1     9     |
 | 4     1     7     | 8     9     3     | 6     5     2     |
 | 5     3     9     | 2     1     6     | 8     4     7     |
 |-------------------+-------------------+-------------------|
 | 36    4     8     | 1     5     9     | 2     7     36    |
 | 27    9     1     | 3     6     27    | 4     8     5     |
 | 2367  67    5     | 4     27    8     | 1     9     36    |
 *-----------------------------------------------------------*


Color on 7s and it's solved.

I'm not sure if this is a valid BUG situation, but I looked at it and eliminated the 2 and 3 from r9c1, which also solved it. BUG+2 ? Somebody knows.

There's also a UR in r79c9 and r79c1 that sets r1c1 and r9c2 = 6. That solves it also.

There's also the xy chain 7-(r9c2)-6-(r3c2)-7-(r3c6)-2-(r1c5)-7 that eliminates the 7 in r9c5, which also solves it.

In nice loop notation:

[r9c5]-7-[r9c2]-6-[r3c2]-7-[r3c6]-2-[r1c5]-7-[r9c5] => r9c5<>7

Of course, if you're a sudoku newbie then none of this will make any sense to you.

All of the techniques above are explained in links in the 4th message in this thread:

http://forum.enjoysudoku.com/viewtopic.php?t=3315
Sped
 
Posts: 126
Joined: 26 March 2006

Another observation

Postby keith » Fri Jun 16, 2006 8:29 pm

There is a remote naked pair on <27> that says R1C1 is not <7>. I am just pointing it out because they are quite rare.

In the diagram, if "a" is <2>, "b" is <7>, and vice-versa.
Code: Select all
*-----------------------------------------------------------*
 | 67    8     3     | 5     27b   1     | 9     26    4     |
 | 9     5     2     | 6     8     4     | 7     3     1     |
 | 1     67    4     | 9     3     27a   | 5     26    8     |
 |-------------------+-------------------+-------------------|
 | 8     2     6     | 7     4     5     | 3     1     9     |
 | 4     1     7     | 8     9     3     | 6     5     2     |
 | 5     3     9     | 2     1     6     | 8     4     7     |
 |-------------------+-------------------+-------------------|
 | 36    4     8     | 1     5     9     | 2     7     36    |
 | 27a   9     1     | 3     6     27b   | 4     8     5     |
 | 2367  67    5     | 4     27a   8     | 1     9     36    |
 *-----------------------------------------------------------*


The BUG reduction is valid. I am not sure what it would be called. (I am guessing most would say a BUG+2 is a pattern where all except two unsolved cells have two candidates.)

The BUG reduction says R9C1 is <67>. The UR says it is <27>. Taken together, R9C1 is solved as <7>.

I know either of them solves the puzzle, but this is a good (and simple) illustration of how the reduction of one pattern can destroy another.

Interesting endgame!

Keith
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