Let G be a finite group. Show that if G has exactly one nontrivial subgroup, then order of G is p^2 for some prime p. Solution Lagrange \'s theorem in group theory states that : ( G ,* ) is a group , His a sub group of G then O (H) divides O(G) using the above fact we conclude that : If O(G) = n then order of the subgroups are the factors of \'n\' ie if 1, a, b , ------n are the distinct factors of \'n then there are subgroups of order 1,a,b, ---n the proper subgroups will have order a,b , there is only one subgroup => the factors of \'n\' are 1,a,n => 1xn=a2 =p2 where \'p \' is a prime no Example n = 9 only one proper subgroup of order =3 n= 25 ---------------- \'\'-----------------------=5 and so on.