I'm stuck

Everything about Sudoku that doesn't fit in one of the other sections

I'm stuck

Postby in_the_everyday » Wed Dec 14, 2005 5:24 pm

Two sudoku puzzles were printed in the school news paper. I found the easy one to be just that, easy. The hard one seems impossible. I wrote down all the possible numbers that could go in each empty square. I even figured out a few things that were obvious, but it still left two possibilities. I've tested situations out where there were two possibilities hoping to find a confound. It didn't work. I eventually just gave up and looked at the answer. Anyway, I was hoping someone on here could demonstrate a proof that would result in progress from where I got stuck. Asteriks represent blanks.

**9 *3* ***
5*6 **8 ***
**7 *4* 61*

7*2 **4 *8*
9*3 *8* 4*7
*84 7** ***

*71 869 3**
*95 4** 8*6
**8 *5* 9*1

All of the numbers above were compared to the answer printed in the paper, and they are correct. Some of them were given and some of them I had no trouble figuring out. If you notice, there's only one 2 up there, and it was one that I ended up figuring out. I realized early on that this one was hard because a 2 could possibly be anywhere from the start.
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Postby Ruud » Wed Dec 14, 2005 5:51 pm

Here is the candidate grid:

Code: Select all
.------------------.------------------.------------------.
| 1248  124   9    | 1256  3     12567| 257   245   2458 |
| 5     1234  6    | 129   1279  8    | 27    2349  2349 |
| 238   23    7    | 259   4     25   | 6     1     23589|
:------------------+------------------+------------------:
| 7     156   2    | 13569 19    4    | 15    8     359  |
| 9     156   3    | 1256  8     1256 | 4     256   7    |
| 16    8     4    | 7     129   12356| 125   23569 2359 |
:------------------+------------------+------------------:
| 24    7     1    | 8     6     9    | 3     245   245  |
| 23    9     5    | 4     127   1237 | 8     27    6    |
| 2346  2346  8    | 23    5     237  | 9     247   1    |
'------------------'------------------'------------------'


There is something hidden in column 8.

Ruud.
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Postby in_the_everyday » Wed Dec 14, 2005 6:00 pm

I came up with the candidate gride by hand, and everything I've checked matches the one you gave. I assume you used a computer program for it.
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Postby in_the_everyday » Wed Dec 14, 2005 6:03 pm

oh, I see... there are only two squares in the column that could contain a 3 or a 9 which would automatically limit the possibilities for those two squares to 3 or 9 rather than the large amount of possibilities that at first seemed possible.
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proving confounds

Postby in_the_everyday » Wed Dec 14, 2005 6:57 pm

Well determining that either one of those two squares could be 3 or 9 revealed a 6, but that didn't amount to much. I then went on to prove that placing 3 in the top square and 9 in the bottom square resulted in confounding numbers further down the road which would mean 9 belongs in the top square and 3 in the bottom square. Is it normal to go to such lengths to figure something out?
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Postby in_the_everyday » Wed Dec 14, 2005 7:03 pm

actually that proof is wrong... must've made a mistake somewhere
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Postby Ruud » Wed Dec 14, 2005 7:19 pm

After you isolated the 3 and 9 in column 8, and placed the 6,
there are locked candidates in the center box.
This in turn reveals something naked in that same box.

Ruud.
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