counting Sudoku _problems_

[N. J. A. Sloane]

Hugo, there has been a lot of discussion
about sudokus on the math fun list.
In particular the following message is relevant to
your question:

> Date: Thu, 21 Jul 2005 12:13:14 +0100
> From: Gary McGuire <Gary.McGuire at nuim.ie>
> 
> 
> I have been spending way too much time on sudoku.
> There are a few interesting questions around, but most have been solved.
> There is one remaining big open question:
> 
> What is the fewest number of clues a sudoku can have?
> 
> The current record is 17.  Here is an example of a puzzle with 17 clues.
> 
> 
> . 9 8     . . .    . . .
> . . .     . 7 .    . . .
> . . .     . 1 5    . . .
> -------+-------+-------
> 1 . .     . . .    . . .
> . . .     2 . .    . . 9
> . . .     9 . 6    . 8 2
> -------+-------+-------
> . . .     . . .    . 3 .
> 5 . 1     . . .    . . .
> . . .     4 . .    . 2 .
> 
> ... (many lines omitted)
> 
> 
> Are there clever ways to search for the set of size 16?
> Is this approach feasible?
> 

NJAS