Sudoku and Graph Theory

When you get stuck on a fiendishly difficult sudoku, it's hard not to wonder if the puzzle really has a solution. At another moment, aglow in the triumph of a clever deduction, you might have a sneaking suspicion that there may be a simpler, more systematic way of finding the answer. Further questions may come to mind: How many different sudoku puzzles are possible in the standard 9-by-9 format? Can a puzzle with few initial entries be easier to solve than one with more entries? What's the smallest number of initial entries necessary to guarantee that there's one, and only one, solution? For Details Click Me