关于数独的组合种类
The number of valid Sudoku solution grids for the standard 9×9 grid was calculated by Bertram Felgenhauer in 2005 to be 6,670,903,752,021,072,936,960 ,This number is equal to 9! × 722 × 27 × 27,704,267,971, the last factor of which is prime.
有什么好的算法可以得到6,670,903,752,021,072,936,960 ?或者27,704,267,971,722 各自怎么来的?从数学的角度看有什么规律?
9! × 722 × 27 应该是同一组合通过旋转,镜像等得到的排列数,但
27,704,267,971 怎么来的?穷举?