Counting and indexing Sudoku solutions
Profesor Watanabe has finalized his method for efficiently index and provide an index number to each Sudoku problem (I wrote about a discussion with him on the topic I had last summer here). The number of different Sudoku problems (and solutions since each problem has only one solution, the reason why it is so interesting) is 6,670,903,752,021,072,936,960 and it was already discovered in 2005 by Bertram Felgenhauer and Frazer Jarvis and now what Watanabe-sensei has discovered is how to efficiently assign a number to each Sudoku problem, a task that obviously cannot be done efficiently looking at each solution due to the huge amount. You can see and try their method here.
CD
