# Definition:Galois Group of Field Extension

## Contents

## Definition

Let $L / K$ be a field extension.

The **Galois group** of $L / K$ is the subgroup of the automorphism group of $L$ consisting of field automorphisms that fix $K$ point-wise:

- $\Gal {L / K} = \set{\sigma \in \Aut L: \forall k \in K: \map \sigma k = k}$

### As a topological group

The notation $\Gal {L / K}$ is also a shorthand for the topological group:

- $\struct {\Gal {L / K}, \tau}$

where $\tau$ is the Krull topology.

### Alternative Definition

More generally, we can abandon the condition that $L / K$ be Galois if we choose an algebraic closure $\overline K$ such that $L \subseteq \overline K$ and define:

- $\Gal {L / K} = \left\{ {\sigma: L \to \overline K: \sigma}\right.$ is an embedding of $L$ such that $\sigma$ fixes $K$ point-wise$\left.\right\}$

This set will form a group if and only if $L / K$ is normal.

## Also denoted as

The **Galois group** of $L / K$ can also be denoted $\map G {L / K}$.

## Also known as

The **Galois group** of $L / K$ is also known as its **automorphism group** and denoted $\Aut {L / K}$.

Some authors refer to $\Aut {L / K}$ only as a **Galois group** when $L / K$ is a Galois extension.

Some sources use the notation $\map G {L \mid K}$.

## Also see

## Source of Name

This entry was named for Évariste Galois.

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next): Entry:**Galois group** - 2008: David Joyner:
*Adventures in Group Theory*(2nd ed.) ... (previous) ... (next): Where to begin... - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next): Entry:**Galois group**