In mathematics, a Galois extension is an algebraic field extension E/F that is normal and separable; or equivalently, E/F is algebraic, and the field fixed by the automorphism group Aut(E/F) is precisely the base field F. One says that such an extension is Galois. The significance of being a Galois extension is that the extension has a Galois group and obeys the fundamental theorem of Galois theory. 
A result of Emil Artin allows one to construct Galois extensions as follows: If E is a given field, and G is a finite group of automorphisms of E with fixed field F, then E/F is a Galois extension.
Characterization of Galois extensions
- E/F is a normal extension and a separable extension.
- E is a splitting field of a separable polynomial with coefficients in F.
- |Aut(E/F)| = [E:F], that is, the number of automorphisms equals the degree of the extension.
Other equivalent statements are:
- Every irreducible polynomial in F[x] with at least one root in E splits over E and is separable.
- |Aut(E/F)| ≥ [E:F], that is, the number of automorphisms is at least the degree of the extension.
- F is the fixed field of a subgroup of Aut(E).
- F is the fixed field of Aut(E/F).
- There is a one-to-one correspondence between subfields of E/F and subgroups of Aut(E/F).
There are two basic ways to construct examples of Galois extensions.
- Take any field E, any subgroup of Aut(E), and let F be the fixed field.
- Take any field F, any separable polynomial in F[x], and let E be its splitting field.
Adjoining to the rational number field the square root of 2 gives a Galois extension, while adjoining the cube root of 2 gives a non-Galois extension. Both these extensions are separable, because they have characteristic zero. The first of them is the splitting field of x2 − 2; the second has normal closure that includes the complex cube roots of unity, and so is not a splitting field. In fact, it has no automorphism other than the identity, because it is contained in the real numbers and x3 − 2 has just one real root. For more detailed examples, see the page on the fundamental theorem of Galois theory
- See the article Galois group for definitions of some of these terms and some examples.
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