counting Sudoku _problems_

[hv at crypt.org]

"N. J. A. Sloane" <njas at research.att.com> wrote:
:> From: Gary McGuire <Gary.McGuire at nuim.ie>
[...]
:> 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 .

For what it's worth, here is a link to a site with multiple 17s:
  http://www.csse.uwa.edu.au/~gordon/sudokumin.php

There are 450 listed there at the time I looked, each checked to
be distinguishable (in the sense of A109741) from the others.

Hugo