<< How hard, on average, is the resulting Sudoku?

Here's another question, perhaps easier to answer:

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. - Scott