Automorphism Group/Examples/Cyclic Group C3/Proof 1
Example of Automorphism Group
Consider the cyclic group $C_3$, which can be presented as its Cayley table:
- $\begin {array} {r|rrr} \struct {\Z_3, +_3} & \eqclass 0 3 & \eqclass 1 3 & \eqclass 2 3 \\ \hline \eqclass 0 3 & \eqclass 0 3 & \eqclass 1 3 & \eqclass 2 3 \\ \eqclass 1 3 & \eqclass 1 3 & \eqclass 2 3 & \eqclass 0 0 \\ \eqclass 2 3 & \eqclass 2 3 & \eqclass 0 3 & \eqclass 1 3 \\ \end {array}$
The automorphism group of $C_3$ is given by:
- $\Aut {C_3} = \set {\phi, \theta}$
where $\phi$ and $\theta$ are defined as:
\(\ds \map \phi {\eqclass 0 3}\) | \(=\) | \(\ds \eqclass 0 3\) | ||||||||||||
\(\ds \map \phi {\eqclass 1 3}\) | \(=\) | \(\ds \eqclass 1 3\) | ||||||||||||
\(\ds \map \phi {\eqclass 2 3}\) | \(=\) | \(\ds \eqclass 2 3\) |
\(\ds \map \theta {\eqclass 0 3}\) | \(=\) | \(\ds \eqclass 0 3\) | ||||||||||||
\(\ds \map \theta {\eqclass 1 3}\) | \(=\) | \(\ds \eqclass 2 3\) | ||||||||||||
\(\ds \map \theta {\eqclass 2 3}\) | \(=\) | \(\ds \eqclass 1 3\) |
The Cayley table of $\Aut {C_3}$ is then:
- $\begin{array}{r|rr}
& \phi & \theta \\
\hline \phi & \phi & \theta \\ \theta & \theta & \phi \\ \end{array}$
Proof
Let $\xi$ be a general automorphism on $C_3$.
Then by Group Homomorphism Preserves Identity we immediately have that:
- $\map \xi {\eqclass 0 3} = \eqclass 0 3$
Investigating $\map \xi {\eqclass 1 3}$, we find $2$ options:
- $\map \xi {\eqclass 1 3} = \eqclass 1 3$
- $\map \xi {\eqclass 1 3} = \eqclass 2 3$
Each leads to one and only one bijection from $C_3$ to $C_3$, that is, $\phi$ and $\theta$ as defined.
It is determined by inspection that both $\phi$ and $\theta$ are automorphisms.
Hence Automorphism Group is Subgroup of Symmetric Group is applied to confirm that $\set {\phi, \theta}$ forms a group.
The Cayley table follows.
$\blacksquare$