Chapter 11
Field Automorphisms
The theme of this chapter is the construction of automorphisms to given speci-
fications. We begin with a generalisation of a K-automorphism, known as a K-
monomorphism. For normal extensions we shall use K-monomorphisms to build up
K-automorphisms. Using this technique, we can calculate the order of the Galois
group of any finite normal extension, which combines with the result of Chapter 10
to give a crucial part of the fundamental theorem of Chapter 12.
We also introduce the concept of a normal closure of a finite extension. This
useful device enables us to steer around some of the technical obstructions caused by
non-normal extensions.
11.1 K-Monomorphisms
We begin by generalising the concept of a K-automorphism of a subfield L of C,
by relaxing the condition that the map should be onto. We continue to require it to be
one-to-one.
Definition 11.1. Suppose that K is a subfield of each of the subfields M and L of C.
Then a K-monomorphism of M into L is a field monomorphism φ : M L such that
φ(k) = k for every k K.
Example 11.2. Suppose that K = Q,M = Q(α) where α is a real cube root of 2, and
L = C. We can define a K-monomorphism φ : M L by insisting that φ(α) = ωα,
where ω = e
2πi/3
. In more detail, every element of M is of the form p + qα + rα
2
where p,q,r Q, and
φ(p + qα + rα
2
) = p + qωα + rω
2
α
2
Since α and ωα have the same minimal polynomial, namely t
3
2, Corollary 5.13
implies that φ is a K-monomorphism.
There are two other K-monomorphisms M L in this case. One is the identity,
and the other takes α to ω
2
α (see Figure 18).
In general if K M L then any K-automorphism of L restricts to a K-
monomorphism M L. We are particularly interested in when this process can be
reversed.
145
146 Field Automorphisms
FIGURE 18: Images of Q-monomorphisms of α = Q(
3
2) : Q.
Theorem 11.3. Suppose that L : K is a finite normal extension and K M L. Let
τ be any K-monomorphism M L. Then there exists a K-automorphism σ of L such
that σ|
M
= τ.
Proof. By Theorem 9.9, L is the splitting field over K of some polynomial f over K.
Hence it is simultaneously the splitting field over M for f and over τ(M) for τ( f ).
But τ|
K
is the identity, so τ( f ) = f . We have the diagram
M L
τ σ
τ(M) L
with σ yet to be found. By Theorem 9.6, there is an isomorphism σ : L L such that
σ|
M
= τ. Therefore σ is an automorphism of L, and since σ |
K
= τ|
K
is the identity,
σ is a K-automorphism of L.
This result can be used to construct K-automorphisms:
Proposition 11.4. Suppose that L : K is a finite normal extension, and α,β are zeros
in L of the irreducible polynomial p over K. Then there exists a K-automorphism σ
of L such that σ(α) = β .
Proof. By Corollary 5.13 there is an isomorphism τ : K(α) K(β ) such that τ|
K
is the identity and τ(α) = β . By Theorem 11.3, τ extends to a K-automorphism σ
of L.
11.2 Normal Closures
When extensions are not normal, we can try to recover normality by making the
extensions larger.
Definition 11.5. Let L be a finite extension of K. A normal closure of L : K is an
extension N of L such that

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