Galois extension
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In mathematics, a Galois extension is a field extension E/F satisfying certain conditions (described below); one also says that the 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.
The definition is as follows. The extension E/F is Galois if the field fixed by the automorphism group Aut(E/F) is precisely the base field F. (See the article Galois group for definitions of some of these terms and some examples.)
A fundamental result of Galois theory states that a finite extension E/F is Galois if and only if either of the following conditions holds:
- E/F is both a normal extension and a separable extension.
- More concretely, that E is the splitting field of a separable polynomial with coefficients in F.
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, then E/F is a Galois extension, where F is the fixed field of G.