# Field automorphism

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In field theory, a field automorphism is an automorphism of the algebraic structure of a field, that is, a bijective function from the field onto itself which respects the fields operations of addition and multiplication.

The automorphisms of a given field K form a group, the automorphism group Aut(K).

If L is a subfield of K, an automorphism of K which fixes every element of L is termed an L-automorphism. The L-automorphisms of K form a subgroup AutL(K) of the full automorphism group of K. A field extension K / L of finite index d is normal if the automorphism group is of order equal to d.

## Examples

• The field Q of rational numbers has only the identity automorphism, since an automorphism must map the unit element 1 to itself, and every rational number may be characterized via 1 and field operations preserved by automorphisms.
• Similarly, a finite field of prime order has only the identity automorphism.
• The field R of real numbers has only the identity automorphism. This is harder to prove, and relies on the fact that R is an ordered field, with a unique ordering defined by the positive real numbers, which are precisely the squares, so that in this case any automorphism must also respect the ordering.
• The field C of complex numbers has two automorphisms, the identity and complex conjugation.
• A finite field Fq of prime power order q, where q = pf is a power of the prime number p, has the Frobenius automorphism, $\Phi: x \mapsto x^p$. The automorphism group in this case is cyclic of order f, generated by Φ.
• The quadratic field $\mathbf{Q}(\sqrt d)$ has a non-trivial automorphism which maps $\sqrt d \mapsto - \sqrt d$. The automorphism group is cyclic of order 2.

A homomorphism of fields is necessarily injective, since it is a ring homomorphism with trivial kernel, and a field, viewed as a ring, has no non-trivial ideals. An endomorphism of a field need not be surjective, however. An example is the Frobenius map $\Phi: x \mapsto x^p$ applied to the rational function field $\mathbf{F}_p(X)$, which has as image the proper subfield $\mathbf{F}_p(X^p)$.