The cyclic groups are the simplest, thus our friends. For example, they are always abelian (but not the converse). And they
have many applications. I discuss many such applications in my book, Fourier Analysis on Finite Groups and Applications.
Many applications come from thinking of large
nite cyclic groups as good approximations for a circle. The fast Fourier
transform is one such application. It allows rapid signal and data processing.
Other groups are more complicated, especially the non-abelian ones. J. H. Conway says that groups are adept at doing
large numbers of impossible things before breakfast.
What does it mean for a group G under multiplication to be cyclic?