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Field theory is the branch of mathematics in which fields are studied. This is a glossary of some terms of the subject. (See field theory (physics) for the unrelated field theories in physics.)

### Definition of a field

A field is a commutative ring (F,+,*) in which 0≠1 and every nonzero element has a multiplicative inverse. In a field we thus can perform the operations addition, subtraction, multiplication, and division.

The non-zero elements of a field F form an abelian group under multiplication; this group is typically denoted by F×;

The ring of polynomials in the variable x with coefficients in F is denoted by F[x].

### Basic definitions

Characteristic
The characteristic of the field F is the smallest positive integer n such that n·1 = 0; here n·1 stands for n summands 1 + 1 + 1 + ... + 1. If no such n exists, we say the characteristic is zero. Every non-zero characteristic is a prime number. For example, the rational numbers, the real numbers and the p-adic numbers have characteristic 0, while the finite field Zp has characteristic p.
Subfield
A subfield of a field F is a subset of F which is closed under the field operation + and * of F and which, with these operations, forms itself a field.
Prime field
The prime field of the field F is the unique smallest subfield of F.
Extension field
If F is a subfield of E then E is an extension field of F. We then also say that E/F is a field extension.
Degree of an extension
Given an extension E/F, the field E can be considered as a vector space over the field F, and the dimension of this vector space is the degree of the extension, denoted by [E : F].
Finite extension
A finite extension is a field extension whose degree is finite.
Algebraic extension
If an element α of an extension field E over F is the root of a non-zero polynomial in F[x], then α is algebraic over F. If every element of E is algebraic over F, then E/F is an algebraic extension.
Generating set
Given a field extension E/F and a subset S of E, we write F(S) for the smallest subfield of E that contains both F and S. It consists of all the elements of E that can be obtained by repeatedly using the operations +,-,*,/ on the elements of F and S. If E = F(S) we say that E is generated by S over F.
Primitive element
An element α of an extension field E over a field F is called a primitive element if E=F(α), the smallest extension field containing α. Such an extension is called a simple extension.
Splitting field
A field extension generated by the complete factorisation of a polynomial.
Normal extension
A field extension generated by the complete factorisation of a set of polynomials.
Separable extension
An extension generated by roots of separable polynomials.
Perfect field
A field such that every finite extension is separable. All fields of characteristic zero, and all finite fields, are perfect.
Imperfect degree
Let F be a field of characteristic p>0; then Fp is a subfield. The degree [F:Fp] is called the imperfect degree of F. The field F is perfect if and only if its imperfect degree is 1. For example, if F is a function field of n variables over a finite field of characteristic p>0, then its imperfect degree is pn.
Algebraically closed field
A field F is algebraically closed if every polynomial in F[x] has a root in F; equivalently: every polynomial in F[x] is a product of linear factors.
Algebraic closure
An algebraic closure of a field F is an algebraic extension of F which is algebraically closed. Every field has an algebraic closure, and it is essentially unique.
Transcendental
Those elements of an extension field of F that are not algebraic over F are transcendental over F.
Algebraically independent elements
Elements of an extension field of F are algebraically independent over F if they don't satisfy any polynomial equation with coefficients in F.
Transcendence degree
The number of algebraically independent transcendental elements in a field extension. It is used to define the dimension of an algebraic variety.

### Homomorphisms

Field homomorphism
A field homomorphism between two fields E and F is a function
f : EF
such that
f(x + y) = f(x) + f(y)
and
f(xy) = f(x) f(y)
for all x, y in E, as well as f(1) = 1. These properties imply that f(0) = 0, f(x-1) = f(x)-1 for x in E with x ≠ 0, and that f is injective. Fields, together with these homomorphisms, form a category. Two fields E and F are called isomorphic if there exists a bijective homomorphism
f : EF.
The two fields are then identical for all practical purposes; however, not necessarily in a unique way. See, for example, complex conjugation.

### Types of fields

Finite field
A field with finitely many elements.
Ordered field
A field with a total order compatible with its operations.
Rational numbers
Real numbers
Complex numbers
Number field
Finite extension of the field of rational numbers.
Algebraic numbers
The field of algebraic numbers is the smallest algebraically closed extension of the field of rational numbers. Their detailed properties are studied in algebraic number theory.
A degree-two extension of the rational numbers.
Cyclotomic field
An extension of the rational numbers generated by a root of unity.
Totally real field
A number field generated by a root of a polynomial, having all its roots real numbers.
Formally real field
Real closed field
Global field
A number field or a function field of one variable over a finite field.
Local field
A completion of some global field (w.r.t. a prime of the integer ring).

### Galois theory

Galois extension
A normal, separable field extension.
Galois group
The automorphism group of a Galois extension. When it is a finite extension, this is a finite group of order equal to the degree of the extension. Galois groups for infinite extensions are profinite groups.
Kummer theory
The Galois theory of taking n-th roots, given enough roots of unity. It includes the general theory of quadratic extensions.
Artin-Schreier theory
Covers an exceptional case of Kummer theory, in characteristic p.
Normal basis
A basis in the vector space sense of L over K, on which the Galois group of L over K acts transitively.
Tensor product of fields
A different foundational piece of algebra, including the compositum operation (join of fields).

### Extensions of Galois theory

Inverse problem of Galois theory
Given a group G, find an extension of the rational number or other field with G as Galois group.
Differential Galois theory
The subject in which symmetry groups of differential equations are studied along the lines traditional in Galois theory. This is actually an old idea, and one of the motivations when Sophus Lie founded the theory of Lie groups. It has not, probably, reached definitive form.
Grothendieck's Galois theory
A very abstract approach from algebraic geometry, introduced to study the analogue of the fundamental group.

Published - March 2009

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