Let be a group such that any is of order or , meaning that where is the neutral.
- Show that is abelian.
Now we assume that is finite. - Show that can be generated by a finite family.
- Show that there exists a group isomorphism between and some where is a positive integer.
PS: We recommand the user to not use any bazooka like Fröbenius classification theorem (which says that any finite abelian group is isomorphic to a product of where is prime) or Cauchy lemma (which says that if prime divides then there exists an element of order in ).
