Definition:Group of Outer Automorphisms

Definition
Let $G$ be a group.

Let $\Aut G$ be the automorphism group of $G$.

Let $\Inn G$ be the inner automorphism group of $G$.

Let $\dfrac {\Aut G} {\Inn G}$ be the quotient group of $\Aut G$ by $\Inn G$.

Then $\dfrac {\Aut G} {\Inn G}$ is called the group of outer automorphisms of $G$.

This group $\dfrac {\Aut G} {\Inn G}$ is often denoted $\Out G$.