*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?

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 '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)

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**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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