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Puzzles with only 18 clues.

 
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zoltag
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Joined: 01 Apr 2006
Posts: 126

PostPosted: Tue May 15, 2007 1:58 am    Post subject: Puzzles with only 18 clues. Reply with quote

Do you have a list of 18 clue minimal puzzles that you would share?

There is so much attention on 17s, yet there should be about 3 million 18s.

Anyone have a list of some?

I have seen a list of about 100, there are for sure way more.
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Ruud
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Joined: 30 Dec 2005
Posts: 601

PostPosted: Tue May 15, 2007 8:24 pm    Post subject: Reply with quote

Gordon F Royle had a list of 18 clue sudokus on his website, but took it down, probably because it cost him a lot of bandwidth to have such a large file available for download.

There is a relatively easy way to create a large series of 18 clue sudokus:

For each 17-clue sudoku in Gordon's collection:

remove 1 clue at random, then add 2 clues at random. Check for a unique solution and whether the puzzle is minimal. If it is not, you might be able to find a new 17, but this is highly unlikely, because several people have already performed an exhaustive "remove 1-add 1" search on the collection.
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zoltag
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PostPosted: Tue May 15, 2007 10:40 pm    Post subject: Reply with quote

Thanks Ruud!
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Glyn
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Joined: 16 Jan 2007
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PostPosted: Tue May 15, 2007 11:45 pm    Post subject: Reply with quote

If you have a look over at the Players Forum you'll find an ongoing discussion about 17/18 clue puzzles under General/Puzzles. Gordon Royle and others are posting info on that forum so you can fill in a bit more on the techniques that Ruud mentioned and the latest findings.
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