In mathematics, the **fundamental theorem of Galois theory** is a result that describes the structure of certain types of field extensions in relation to groups. It was proved by Évariste Galois in his development of Galois theory.

In its most basic form, the theorem asserts that given a field extension *E*/*F* that is finite and Galois, there is a one-to-one correspondence between its intermediate fields and subgroups of its Galois group. (*Intermediate fields* are fields *K* satisfying *F* ⊆ *K* ⊆ *E*; they are also called *subextensions* of *E*/*F*.)

For finite extensions, the correspondence can be described explicitly as follows.

- For any subgroup
*H*of Gal(*E*/*F*), the corresponding fixed field, denoted*E*, is the set of those elements of^{H}*E*which are fixed by every automorphism in*H*. - For any intermediate field
*K*of*E*/*F*, the corresponding subgroup is Aut(*E*/*K*), that is, the set of those automorphisms in Gal(*E*/*F*) which fix every element of*K*.

The fundamental theorem says that this correspondence is a one-to-one correspondence if (and only if) *E*/*F* is a Galois extension.
For example, the topmost field *E* corresponds to the trivial subgroup of Gal(*E*/*F*), and the base field *F* corresponds to the whole group Gal(*E*/*F*).

The notation Gal(*E*/*F*) is only used for Galois extensions. If *E*/*F* is Galois, then Gal(*E*/*F*) = Aut(*E*/*F*). If *E*/*F* is not Galois, then the "correspondence" gives only an injective (but not surjective) map from to , and a surjective (but not injective) map in the reverse direction. In particular, if *E*/*F* is not Galois, then *F* is not the fixed field of any subgroup of Aut(*E*/*F*).

The correspondence has the following useful properties.

- It is
*inclusion-reversing*. The inclusion of subgroups*H*_{1}⊆*H*_{2}holds if and only if the inclusion of fields*E*^{H1}⊇*E*^{H2}holds. - Degrees of extensions are related to orders of groups, in a manner consistent with the inclusion-reversing property. Specifically, if
*H*is a subgroup of Gal(*E*/*F*), then |*H*| = [*E*:*E*] and |Gal(^{H}*E*/*F*)|/|*H*| = [*E*:^{H}*F*]. - The field
*E*is a normal extension of^{H}*F*(or, equivalently, Galois extension, since any subextension of a separable extension is separable) if and only if*H*is a normal subgroup of Gal(*E*/*F*). In this case, the restriction of the elements of Gal(*E*/*F*) to*E*induces an isomorphism between Gal(^{H}*E*/^{H}*F*) and the quotient group Gal(*E*/*F*)/*H*.

Consider the field

Since *K* is constructed from the base field by adjoining √2, then √3, each element of *K* can be written as:

Its Galois group comprises the automorphisms of *K* which fix *a*. Such automorphisms must send √2 to √2 or –√2, and send √3 to √3 or –√3, since they permute the roots of any irreducible polynomial. Suppose that *f* exchanges √2 and –√2, so

and *g* exchanges √3 and –√3, so

These are clearly automorphisms of *K*, respecting its addition and multiplication. There is also the identity automorphism *e* which fixes each element, and the composition of *f* and *g* which changes the signs on *both* radicals:

Since the order of the Galois group is equal to the degree of the field extension, , there can be no further automorphisms:

which is isomorphic to the Klein four-group. Its five subgroups correspond to the fields intermediate between the base and the extension *K*.

- The trivial subgroup {1} corresponds to the entire extension field
*K*. - The entire group
*G*corresponds to the base field - The subgroup {1,
*f*} corresponds to the subfield since*f*fixes √3. - The subgroup {1,
*g*} corresponds to the subfield since*g*fixes √2. - The subgroup {1,
*fg*} corresponds to the subfield since*fg*fixes √6.

The following is the simplest case where the Galois group is not abelian.

Consider the splitting field *K* of the irreducible polynomial over ; that is, where *θ* is a cube root of 2, and *ω* is a cube root of 1 (but not 1 itself). If we consider *K* inside the complex numbers, we may take , the real cube root of 2, and Since *ω* has minimal polynomial *,* the extension has degree:

with -basis as in the previous example. Therefore the Galois group has six elements, determined by all permutations of the three roots of :

Since there are only 3! = 6 such permutations, *G* must be isomorphic to the symmetric group of all permutations of three objects. The group can be generated by two automorphisms *f* and *g* defined by:

and , obeying the relations . Their effect as permutations of is (in cycle notation): . Also, *g* can be considered as the complex conjugation mapping.

The subgroups of *G* and corresponding subfields are as follows:

- As always, the trivial group {1} corresponds to the whole field
*K*, while the entire group*G*to the base field .

- The unique subgroup of order 3, , corresponds to the subfield of degree two, since the subgroup has index two in
*G*: i.e. . Also, this subgroup is normal, so the subfield is normal over , being the splitting field of . Its Galois group over the base field is the quotient group , where [*g*] denotes the coset of*g*modulo*H*; that is, its only non-trivial automorphism is the complex conjugation*g*.

- There are three subgroups of order 2, and corresponding respectively to the subfields These subfields have degree 3 over since the subgroups have index 3 in
*G*. The subgroups are*not*normal in*G*, so the subfields are*not*Galois or normal over . In fact, each subfield contains only a single one of the roots , so none has any non-trivial automorphisms.

Let be the field of rational functions in the indeterminate *λ,* and consider the group of automorphisms:

here we denote an automorphism by its value , so that . This group is isomorphic to (see: six cross-ratios). Let be the fixed field of , so that .

If is a subgroup of , then the coefficients of the polynomial

generate the fixed field of . The Galois correspondence implies that every subfield of can be constructed this way. For example, for , the fixed field is and if then the fixed field is . The fixed field of is the base field where j is the j-invariant written in terms of the modular lambda function:

Similar examples can be constructed for each of the symmetry groups of the platonic solids as these also have faithful actions on the projective line and hence on .

The theorem classifies the intermediate fields of *E*/*F* in terms of group theory. This translation between intermediate fields and subgroups is key
to showing that the general quintic equation is not solvable by radicals (see Abel–Ruffini theorem). One first determines the Galois groups of radical extensions (extensions of the form *F*(α) where α is an *n*-th root of some element of *F*), and then uses the fundamental theorem to show that solvable extensions correspond to solvable groups.

Theories such as Kummer theory and class field theory are predicated on the fundamental theorem.

Given an infinite algebraic extension we can still define it to be Galois if it is normal and separable. The problem that one encounters in the infinite case is that the bijection in the fundamental theorem does not hold as we get too many subgroups generally. More precisely if we just take every subgroup we can in general find two different subgroups that fix the same intermediate field. Therefore we amend this by introducing a topology on the Galois group.

Let be a Galois extension (possibly infinite) and let be the Galois group of the extension. Let

be the set of the Galois groups of all finite intermediate Galois extensions. Note that for all we can define the maps by . We then define the

Now that we have defined a topology on the Galois group we can restate the fundamental theorem for infinite Galois extensions.

Let denote the set of all intermediate field extensions of and let denote the set of all closed subgroups of endowed with the Krull topology. Then there exists a bijection between and given by the map

defined by and the map

defined by . One important thing one needs to check is that is a well-defined map, that is that is a closed subgroup of for all intermediate fields . This is proved in Ribes–Zalesskii, Theorem 2.11.3. ^{[1]}