Originally Posted by

**Bernhard** Dummit and Foote Section 4.4 Automorphisms Exercise 1 reads as follows:

Let $\displaystyle \sigma \in Aut(G)$ and $\displaystyle \phi_g$ is conjugation by g, prove that $\displaystyle \sigma \phi_g \sigma^{-1}$ = $\displaystyle \phi_{\sigma (g)}$.

**Deduce that Inn(G) is a normal subgroup of Aut(G)**

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A start to the proof is as follows:

$\displaystyle \sigma \phi_g \sigma^{-1}$

= $\displaystyle \sigma (\phi_g (\sigma^{-1} (x)))$

= $\displaystyle \sigma (g. \sigma^{-1} (x). g^{-1} )$

= $\displaystyle \sigma (g). x . \sigma (g^{-1} )$

= $\displaystyle \sigma (g). x . {[\sigma (g)]}^{-1}$

Thus $\displaystyle \sigma \phi_g \sigma^{-1}$ = $\displaystyle \phi_{\sigma (g)}$.

My question is - **how do we use this result to show Inn(G) is a normal subgroup of Aut(G)?**

Can someone please help?

Peter