If is a group and is an element of , then the function is called conjugation by . This function is an endomorphism of : for all where the second equality is given by the insertion of the identity between and Furthermore, it has a left and rightinverse, namely Thus, is bijective, and so an isomorphism of with itself, i.e. an automorphism. An inner automorphism is any automorphism that arises from conjugation. When discussing right conjugation, the expression is often denoted exponentially by This notation is used because composition of conjugations satisfies the identity: for all This shows that conjugation gives a right action of on itself.
The composition of two inner automorphisms is again an inner automorphism, and with this operation, the collection of all inner automorphisms of is a group, the inner automorphism group of denoted. is a normal subgroup of the full automorphism group of. The outer automorphism group, is the quotient group The outer automorphism group measures, in a sense, how many automorphisms of are not inner. Every non-inner automorphism yields a non-trivial element of, but different non-inner automorphisms may yield the same element of. Saying that conjugation of by leaves unchanged is equivalent to saying that and commute: 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. An automorphism of a group is inner if and only if it extends to every group containing. By associating the element with the inner automorphism in as above, one obtains an isomorphism between the quotient group and the inner automorphism group: This is a consequence of the first isomorphism theorem, because is precisely the set of those elements of that give the identity mapping as corresponding inner automorphism.
Non-inner automorphisms of finite -groups
A result of Wolfgang Gaschütz says that if is a finite non-abelian-group, then has an automorphism of -power order which is not inner. It is an open problem whether every non-abelian -group has an automorphism of order. The latter question has positive answer whenever has one of the following conditions:
The inner automorphism group of a group,, is trivial if and only if is abelian. The group 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 elements when is not 2 or 6. When, the symmetric group has a unique non-trivial class of outer automorphisms, and when, the symmetric group, despite having no outer automorphisms, is abelian, giving a non-trivial center, disqualifying it from being complete. If the inner automorphism group of a perfect group is simple, then is called quasisimple.
An automorphism of a Lie algebra is called an inner automorphism if it is of the form, where is the adjoint map and is an element of a Lie group whose Lie algebra is. 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 is the group of units of a ring,, then an inner automorphism on can be extended to a mapping on the projective line over by the group of units of the matrix ring,. In particular, the inner automorphisms of the classical groups can be extended in that way.
