How many different Sudoku grids are there?
An enormous number! Apparently the 21 digit number 6,670,903,752,021,072,936,960 – see this technical report if you really want to know why! Many Sudoku published puzzles are symmetric about a rotation of 180 degrees, but they don’t have to be (like ours!). There are many programs around to solve Sudoku (some links below). It’s something a computer can do really well by brute force (in particular a back-tracking algorithm can be programmed quite easily). There’s much more fun in trying yourself to use more satisfying logical methods. There’s a jungle of information on Sudoku on the web – here are some sites, with plenty of puzzles (potentially 6.67 x 10^21 of them!).
