http://www.physicsforums.com/showthread.php?t=473890 에 의하면 (n,m) 개
http://www.physicsforums.com/archive/index.php/t-217416.html
http://www.math.niu.edu/~beachy/courses/421/rev_gps.pdf
http://math.stackexchange.com/questions/74629/homomorphisms-between-z-n-and-z-m
