# Thread: automorphism group of a cyclic group

1. ## automorphism group of a cyclic group

Hi guys,

Is the automorphism group of a cyclic group always abelian?

I believe so but I need to prove it. I don't really know where to start...any ideas?

2. Hi

Let $(G,.)$ be a cyclic group, $a$ one of its generators. Two automorphisms $\phi$ and $\psi$ are determined by $\phi(a)=a^k$ and $\psi(a)=a^l$ for some $k,l \in\mathbb{Z}.$

Since $\psi\circ\phi$ and $\phi\circ\psi$ are automorphisms, they're also determined by $\psi\circ\phi(a)$ and $\phi\circ\psi(a),$ therefore if $\psi\circ\phi(a)=\phi\circ\psi(a)$ then $\psi\circ\phi=\phi\circ\psi$ and the automorphism group is abelian.

So is $\psi\circ\phi(a)=\phi\circ\psi(a)$ true?

3. Originally Posted by ziggychick
Hi guys,

Is the automorphism group of a cyclic group always abelian?

I believe so but I need to prove it. I don't really know where to start...any ideas?
You can do better that what clic-clac said if you want to try and additional exercise.

If $|G|=n$ is a cyclic group then $\text{Aut}(G) \simeq \mathbb{Z}_n^{\times}$.
If $|G|=\infty$ then $\text{Aut}(G) \simeq \mathbb{Z}_2$.

4. oh i see now. i can prove those two things.

thanks!