Thanks, Scott.
From the link you provided I see that another interesting question is, What is the smallest number n such that *every* sudoku solution can be reduced to n entries having that solution as its unique completion?
Also: Is there a consensus about how to evaluate the "difficulty" of a given sudoku puzzle? (I was suprised to read at that URL that some feel that a large proportion of puzzles of least known size -- 17 -- are "easy".) --Dan ---------------------------------------------------------------------------------------- <<
What is the smallest number of filled squares that can uniquely determine an extension filling all 81 squares?
--Dan
There are many 17-clue sudokus known. The jury is still out whether a 16-clue sudoku exists. Specific sudoku grids which require 17+ clues are known. If you require rotational symmetry of initial clue positions I think 18-clues in the minumum known. Most of the 17-clue sudokus are not very difficult, so globally minimal clue count doesn't seem to correlate with difficulty. See http://www.sudoku.com/forums/viewtopic.php?t=605 for an extensive discussion.