- 24 Dec '04 14:41 / 2 editsSu Doku is a Japanese expression, which, I gather, translates to 'Number Place'. It is also the title of a time-passing quelque chose à faire that is probably more enjoyable than chatting with the aged relatives over for their Christmas visit.

Here is a challenging example (simple premise, difficult to do): http://images.thetimes.co.uk/TGD/picture/0,,159710,00.jpg

Anyone who solves it within 20 minutes will be doing very well indeed. - 24 Dec '04 23:41

Good good.*Originally posted by Acolyte***Done it! Took a while, though, as I keep making mistakes on these things.**

I got two very similar solutions - are there more?

They're surprisingly addictive - here is today's from The Times: http://www.timesonline.co.uk/article/0,,18209-1414243,00.html

It was rated as - Difficulty: Fiendish, and "the most gruelling one yet".

As for multiple solutions, I'm not sure. Multiple solutions clearly exist for some of the puzzles, but I wonder if there is a 'properly unique' solution assuming one follows the only logical path available (if there is such a thing). This seems to be what the reply to the first question below suggests: (lifted from The Times newspaper)

-------------------------------**How do you make sure that a particular puzzle has a unique solution?**Helen Restall, Guildford

Every one of my Su doku puzzles can be solved using logic alone. In practice some people may solve them using guesses or trial and error, but regardless, every puzzle is capable of being solved with logic.

This means the solver should be able to say, every time he or she enters a number in the grid, “I can prove that the number I am entering must go in this cell, and that no other number can go in this cell.”

If you can honestly say that about every number you enter, then all the cells of the grid contain numbers that can go nowhere else. If each cell in the grid is “uniquely correct” (to use a dubious phrase), then the grid as a whole must be.

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Another interesting question:

**I wondered how many possible different sets of final numbers there are in the 9x9 grid. I have calculated a figure but I think it is too large. Has anyone determined this figure?**David Towers, Nottingham

I have seen attempts to do this on Japanese websites, but as my Japanese is not very good I was not able to follow along. It became apparent, however, that there were just so many zeroes that one lost touch with the reality of how big the numbers were. Also, it seems that there are differences of opinion as to how the number should be calculated.

Consider, however, the number of possible solutions. That’s a huge number for a start. Then consider the number of puzzles which can be derived from each of the possible solutions...

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- 25 Dec '04 13:59

Done yesterday's one now. After looking more carefully, I think there's only one solution to the first one, as it should be.*Originally posted by T1000***Good good.**

They're surprisingly addictive - here is today's from The Times: http://www.timesonline.co.uk/article/0,,18209-1414243,00.html

It was rated as - Difficulty: Fiendish, and "the most gruelling one yet".

As for multiple solutions, I'm not sure. Multiple solutions clearly exist for some of the puzzles, but I wonder if there is a 'proper ...[text shortened]... hich can be derived from each of the possible solutions...

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Now here's a question to ponder: there are oodles of solutions to the 'empty' Su Doku puzzle (ie no numbers filled in at the start). But what is the minimum number of boxes that need to be filled in to produce a Su Doku with a unique solution? - 27 Dec '04 16:05 / 1 editHmmm, the Su Dokus in the Times seem to be what I was doing on the flight home, although I have no idea how many hints there need be to make a unique solution. I don't buy the first argument quoted by T1000 though.

All of this helps explain why (a Japanese friend and) I call my lectures the Clore Lecture Theater Co-Prosperity Sphere. - 28 Dec '04 13:55I blame T1000 for two nights of next to no sleep. These things torment me. I have pages and pages of working for them. The numbers 1-9 with crossouts and highlights. Then I found a page that I had been working on that, at some point that I can't remember, I wrote "It won't fit! Why won't it fit?" in huge letters across. They drive me insane.
- 28 Dec '04 14:10

Sounds like you were working on them on acid or something .*Originally posted by XanthosNZ***I blame T1000 for two nights of next to no sleep. These things torment me. I have pages and pages of working for them. The numbers 1-9 with crossouts and highlights. Then I found a page that I had been working on that, at some point that I can't remember, I wrote "It won't fit! Why won't it fit?" in huge letters across. They drive me insane.** - 29 Dec '04 09:31

You don't need pages of working - just draw a 9x9 grid, write all 9 numbers in each cell, and cross them out as you go along. It's probably easier if you do it on a computer so you don't have crossings-out everywhere.*Originally posted by XanthosNZ***I blame T1000 for two nights of next to no sleep. These things torment me. I have pages and pages of working for them. The numbers 1-9 with crossouts and highlights. Then I found a page that I had been working on that, at some point that I can't remember, I wrote "It won't fit! Why won't it fit?" in huge letters across. They drive me insane.** - 29 Dec '04 11:20

My mind doesn't work in an ordered fashion. I would have the grid written out and fill it in as I go along and all over the place check a cell by writing down the numbers 1-9 and crossing out if they can't be in that cell. If it's 3 possibilities or less or a number must exist in one of two cells then I pencil it in.*Originally posted by Acolyte***You don't need pages of working - just draw a 9x9 grid, write all 9 numbers in each cell, and cross them out as you go along. It's probably easier if you do it on a computer so you don't have crossings-out everywhere.**