Sudoku

>> What is the minimal number of digits guaranteeing a unique solution?

The mallest known number of clues a sudoku puzzle can have and still be solvable is 17. Gordon Royle maintains a database of all known puzzles, 48862 so far, but has yet to find a 16 clue puzzle.

Minimum Sudoku

http://units.maths.uwa.edu.au/~gordon/sudokumin.php

Sudoku Fridays

http://www.greatmathsteachingideas.com/2010/06/15/sudoku-fridays/