# Glossary of field theory

Field theory is the branch of mathematics in which fields are studied. This is a glossary of some terms of the subject.

## Definition of a field

A field is an commutative ring (F,+,*) of which every nonzero element is invertible. Over a field, we can perform addition, subtraction, multiplication and division.

The abelian group of non-zero elements of a field F is typically denoted by F×;

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.

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

## Basic definitions

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
A prime field is the unique smallest subfield of F.
Extension field
If F is a subfield of E then E is an extension field of F.
Algebraic extension
If an element α of an extension field E over F is the root of a polynomial in F[x], then α is algebraic over F. If every element of E is algebraic over F, then E is an algebraic extension of F.
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.
Primitive element
A 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.
Perfect field
A field such that every finite extension is separable. All fields of characteristic zero, and all finite fields, are perfect.
Algebraically closed field
A maximal algebraic extension field of F is its algebraic closure. A field is algebraically closed if it is its own algebraic closure.
Transcendental
If an element is not algebraic over F, then it is transcendental.
Transcendence degree
The number of 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 of finitely many elements.
Ordered field
A field with a total order compatible with its operations.
Number field
Algebraic 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

## 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.