How many different sudoku puzzles are there?

Question

The answer: 6, 670, 903, 752, 021, 072, 936, 960, according to this site:

http://www.technologyreview.com/view/426554/mathematicians-solve-minimum-sudoku-problem/

I have tried to get this number using direct methods but basically I have found the question too hard. There are too many possibilities, but I am likely missing some strategic ways to solve the problem. Any help would be greatly appreciated, thanks.

Answer 1

http://www.afjarvis.staff.shef.ac.uk/sudoku/sudoku.pdf ${}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}$

Answer 2

Try Wikipedia for an introduction and overview and additional references.