In abstract algebra an inner automorphism is an automorphism of a group, ring, or algebra given by the conjugation action of a fixed element, called the conjugating element. They can be realized via simple operations from within the group itself, hence the adjective "inner". These inner automorphisms form a subgroup of the automorphism group, and the quotient of the automorphism group by this subgroup is defined as the outer automorphism group.

Definition

If G is a group and g is an element of G (alternatively, if G is a ring, and g is a unit), then the function

${\displaystyle {\begin{aligned}\varphi _{g}\colon G&\to G\\\varphi _{g}(x)&:=g^{-1}xg\end{aligned))}$

is called (right) conjugation by g (see also conjugacy class). This function is an endomorphism of G: for all ${\displaystyle x_{1},x_{2}\in G,}$

${\displaystyle \varphi _{g}(x_{1}x_{2})=g^{-1}x_{1}x_{2}g=g^{-1}x_{1}\left(gg^{-1}\right)x_{2}g=\left(g^{-1}x_{1}g\right)\left(g^{-1}x_{2}g\right)=\varphi _{g}(x_{1})\varphi _{g}(x_{2}),}$

where the second equality is given by the insertion of the identity between ${\displaystyle x_{1))$ and ${\displaystyle x_{2}.}$ Furthermore, it has a left and right inverse, namely ${\displaystyle \varphi _{g^{-1)).}$ Thus, ${\displaystyle \varphi _{g))$ is bijective, and so an isomorphism of G with itself, i.e. an automorphism. An inner automorphism is any automorphism that arises from conjugation.^[1]

When discussing right conjugation, the expression ${\displaystyle g^{-1}xg}$ is often denoted exponentially by ${\displaystyle x^{g}.}$ This notation is used because composition of conjugations satisfies the identity: ${\displaystyle \left(x^{g_{1))\right)^{g_{2))=x^{g_{1}g_{2))}$ for all ${\displaystyle g_{1},g_{2}\in G.}$ This shows that right conjugation gives a right action of G on itself.

Inner and outer automorphism groups

The composition of two inner automorphisms is again an inner automorphism, and with this operation, the collection of all inner automorphisms of G is a group, the inner automorphism group of G denoted Inn(G).

Inn(G) is a normal subgroup of the full automorphism group Aut(G) of G. The outer automorphism group, Out(G) is the quotient group

${\displaystyle \operatorname {Out} (G)=\operatorname {Aut} (G)/\operatorname {Inn} (G).}$

The outer automorphism group measures, in a sense, how many automorphisms of G are not inner. Every non-inner automorphism yields a non-trivial element of Out(G), but different non-inner automorphisms may yield the same element of Out(G).

Saying that conjugation of x by a leaves x unchanged is equivalent to saying that a and x commute:

${\displaystyle a^{-1}xa=x\iff xa=ax.}$

Therefore the existence and number of inner automorphisms that are not the identity mapping is a kind of measure of the failure of the commutative law in the group (or ring).

An automorphism of a group G is inner if and only if it extends to every group containing G.^[2]

By associating the element a ∈ G with the inner automorphism f(x) = x^a in Inn(G) as above, one obtains an isomorphism between the quotient group G / Z(G) (where Z(G) is the center of G) and the inner automorphism group:

${\displaystyle G\,/\,\mathrm {Z} (G)\cong \operatorname {Inn} (G).}$

This is a consequence of the first isomorphism theorem, because Z(G) is precisely the set of those elements of G that give the identity mapping as corresponding inner automorphism (conjugation changes nothing).

Non-inner automorphisms of finite p-groups

A result of Wolfgang Gaschütz says that if G is a finite non-abelian p-group, then G has an automorphism of p-power order which is not inner.

It is an open problem whether every non-abelian p-group G has an automorphism of order p. The latter question has positive answer whenever G has one of the following conditions:

1. G is nilpotent of class 2
2. G is a regular p-group
3. G / Z(G) is a powerful p-group
4. The centralizer in G, C_G, of the center, Z, of the Frattini subgroup, Φ, of G, C_G ∘ Z ∘ Φ(G), is not equal to Φ(G)

Types of groups

The inner automorphism group of a group G, Inn(G), is trivial (i.e., consists only of the identity element) if and only if G is abelian.

The group Inn(G) is cyclic only when it is trivial.

At the opposite end of the spectrum, the inner automorphisms may exhaust the entire automorphism group; a group whose automorphisms are all inner and whose center is trivial is called complete. This is the case for all of the symmetric groups on n elements when n is not 2 or 6. When n = 6, the symmetric group has a unique non-trivial class of non-inner automorphisms, and when n = 2, the symmetric group, despite having no non-inner automorphisms, is abelian, giving a non-trivial center, disqualifying it from being complete.

If the inner automorphism group of a perfect group G is simple, then G is called quasisimple.

Lie algebra case

An automorphism of a Lie algebra 𝔊 is called an inner automorphism if it is of the form Ad_g, where Ad is the adjoint map and g is an element of a Lie group whose Lie algebra is 𝔊. The notion of inner automorphism for Lie algebras is compatible with the notion for groups in the sense that an inner automorphism of a Lie group induces a unique inner automorphism of the corresponding Lie algebra.

Extension

If G is the group of units of a ring, A, then an inner automorphism on G can be extended to a mapping on the projective line over A by the group of units of the matrix ring, M₂(A). In particular, the inner automorphisms of the classical groups can be extended in that way.