# Non trivial Automorphism [duplicate]

Prove that every finite group having more than two elements has a nontrivial Automorphism.

It is from Topics in Algebra by Herstein. I am not able to solve.

• try $x\mapsto a^{-1}xa$.
– user59671
May 14, 2013 at 14:29
• Do you see how to solve this when some element has order $>2$? May 14, 2013 at 14:30
• @CutieKrait, great unless $G$ is abelian ... :) May 14, 2013 at 14:32
• @TedShifrin: then try $x\mapsto x^p$ for an appropriate $p$.
– user59671
May 14, 2013 at 14:42
• @CutieKrait, great unless every element of $G$ has order $2$ ... :) May 14, 2013 at 17:31

Suppose that $G$ is abelian with exponent greater than 2 (i.e. not every element has order 2). Then mapping $g\in G$ to $g^{-1}$ is an automorphism. It's clearly bijective, and since $G$ is abelian it is indeed a homomorphism.
If $G$ is abelian with exponent 2, then it is a vector space over $\mathbb Z/2$. Choose a basis for this vector space, and pick your favourite two basis vectors to interchange. This is a nontrivial automorphism.
If $G$ is not abelian, then the center of $G$, $Z(G)\neq G$. Now, let $G$ act on itself via conjugation. These are all automorphisms. Convince yourself that if $Z(G)\neq G$, there is a nontrivial one.