Daley Ponderings

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heh - the spam filter worked pretty well, though not quite perfectly, I guess. There were about 10 comments made from the same computer (69.151.106.232) this morning, the first one was alright, although his spelling was atrocious. And then he started swearing and insulting me and the rest of the commenters. Once he saw he was blocked by the spam filter, he started removing words one by one until the spam filter let him in. Presumably, since he went back and read the post each time, the badbehavior plugin didn't stop him from trying to post multiple comments. He eventually got to to four words, and it let him post it, though the comment wasn't all that interesting to read, since he started out so badly, I figured that I might as well delete, though my general policy would be to not moderate people's comments.

I don't know exactly what his point was, no links to any pages, though he did leave a couple email addresses, presumably the last one was real, since it comes from the same network as his computer name.

I wonder how many of his comments were trapped as spam due to more than half of the words being spelled incorrectly.

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Hello everybody, I'm looking for the sudoku with the least amount of already-penned-in numbers. It doesn't have to be the most difficult (although it can't be EASY by definition), it just has to be the most empty one. The two above (from October 9 2006) have only 18 numbers in them, and they are, at least for now, the most empty ones I've found. If anyone knows of a puzzle even emptier, please post its link here. Thnx :)

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Hi. I just found a site where the person has available his collection of 36,628 sudokus of "17 original numbers only". Everyone suspects that 17 is the minimum that will produce a unique solution. Here's the site: http://people.csse.uwa.edu.au/gordon/sudokumin.php Enjoy. Gil Kessler

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The puzzles i have seen here are VERY EASY compared to the hardest known. According to my current rating this one by Ocean is the hardest:
.....1.2.
3...4.5..
...6....7
..2.....1
.8..9..3.
4.....8..
5....2...
.9..3.4..
..67.....
You can find a list of the current known hardest at
http://www.sudoku.com/forums/viewtopic.php?t=4212&start=587

This by Mike Metcalf is the one with the hardest single step:
5.......9
.2.1...7.
..8...3..
.4.7.2...
....5....
.....6.1.
..3...8..
.6...4.2.
9.......5

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I'll have to try your hard ones. I wonder if you actually did the "very easy" puzzles by hand - or you typing them into a computer?

I haven't met anyone who has actually solved the first one by hand yet.

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Well, I agree that the first one ravel posted is the hardest one I have ever seen. I still question whether you did the "VERY EASY" ones by hand or whether you are only comparing on a computer.

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While yours is really easy (just needs a hidden pair), the 2 puzzles by YH definitely are very hard ("very easy" was only meant in comparison to the hardest known). The rating by Sudoku Explainer (http://diuf.unifr.ch/people/juillera/Sudoku/Sudoku.html) for both is 9.0 and this is exactly the same as the hardest has, that i personally solved manually, namely gfroyle's beauty:
6.. ... ..3
.7. .8. .9.
..2 ... 5..
... 3.. ...
.8. .1. .7.
... ..2 ...
..5 ... 1..
.9. .4. .8.
3.. ... ..2

Maria45 and Carcul have solved puzzles manually with a rating of up to 10.0.
The current highest rating by SE is 11.4 (for MM's puzzle above)

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Correction: the 2nd by YH rated 9.5. For puzzles with rating above 9.0 you need very long forcing or contradiction chains.
But the hardest need contradiction chains inside contradiction chains, so they are classes harder.

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If you are interested, how the first puzzle *could* be solved, you can either look at Sudoku Explainers solution or do it like this.
With locked candidates, locked sets, a turbot fish and a unique rectangle you come here:

 *---------------------------------------------------------------*
 | 4      569   1268  | 5789   3    15789  | 25    2567   5679   |
 | 1237   359   12    | 6      127  14579  | 8     2357   34579  |
 | 23678  3569  268   | 45789  278  45789  | 2345  23567  1      |
 |--------------------+--------------------+---------------------|
 | 1236   346   1246  | 78     5    13678  | 23    9      378    |
 | 123    8     59    | 3479   17   13479  | 6     12357  357    |
 | 136    7     59    | 2      168  13689  | 345   1358   3458   |
 |--------------------+--------------------+---------------------|
 | 68     46    468   | 1      9    2      | 7     35     35     |
 | 5      1     3     | 78     678  678    | 9     4      2      |
 | 9      2     7     | 35     4    35     | 1     68     68     |
 *---------------------------------------------------------------*

r4c4=3 => r4c7=2 (=> r1c7=5) => r5c1=2 => r6c1=3 => r6c7=4 => r2c9=4 => r3c7=3 => r2c2=3 => r2c6=5
Then in row 2 there is no 9 left, so r4c4 cannot be 3.
This gives you a pair 78 and the puzzle can be solved with basic methods.

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Thanks for explaining some more.

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oops, i cannot format the grid here. Also the 3 in r4c4 already is eliminated (should be 378). Also note that this solution is much shorter than the one i had manually for gfroyle's beauty.

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I added a <pre> tag to your comment, so the table is presented nicer.

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Do you know where I can find blank sudoku puzzles that I can copy and paste and then still write on it.

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like this?