Let Dn be the dihedral group of symmetries of a regular n-gon, and let Cn Dn be the subgroup of rotations. Let also H Cn be an arbitrary subgroup....

2. Let Dn be the dihedral group of symmetries of a regular n-gon, and let Cn ⊂ Dn be the subgroup of rotations. Let also H ⊂ Cn be an arbitrary subgroup. Prove that H is a normal subgroup of Dn.

Remark: H is a subgroup of Cn, so it is also a subgroup of Dn. Note that H is a normal subgroup of Cn, because Cn is Abelian. This means that ghg−1 ∈ H for any g ∈ Cn. What you need to prove is a stronger result: H is normal in Dn, meaning that ghg−1 ∈ H for any g ∈ Dn, and not only for any g ∈ Cn.

Hint: One possible approach is to show that for any element g ∈ Dn, the set gHg^-1 = {ghg^-1 | h ∈ H} is a subgroup of Cn. What is the order of this subgroup?

What do we know about subgroups of cyclic groups? Another approach is to use geometric arguments to prove that for any reflection g ∈ Dn Cn and any rotation r ∈ Cn, one has grg^-1= r^-1.