In: Advanced Math
Suppose G has precisely one subgroup of order 5, and one subgroup of order 7. What is G?
Again, G = Z35.
Indeed, suppose the subgroup of order 5 is H, and the one of order 7 is K. Then
H ∪ K has
1 + (5 − 1) + (7 − 1) = 11
elements. Choose an element a from G \ (H ∪ K). The order of a is 1, 5, 7 or 35.
We claim the order of a is 35. The order of a is not 1 because it is not the identity.
The order of a is neither 5 nor 7, for otherwise a would generate a subgroup hai of
order 5 or 7, distinct from H or K, and we know there is precisely one subgroup
of order 5 (namely, H), and precisely one subgroup of order 7 (namely, K).
Thus G = <a>, and so G is cyclic.