# Finite field

In mathematics, a finite field or Galois field (so-named in honor of Évariste Galois) is a field that contains a finite number of elements. As with any field, a finite field is a set on which the operations of multiplication, addition, subtraction and division are defined and satisfy certain basic rules. The most common examples of finite fields are given by the p when p is a prime number.

Finite fields are fundamental in a number of areas of mathematics and computer science, including number theory, algebraic geometry, Galois theory, finite geometry, cryptography and coding theory.

## Definitions, first examples, and basic properties

The number of elements of a finite field is called its order. A finite field of order q exists if and only if the order q is a prime power pk (where p is a prime number and k is a positive integer). All finite fields of a given order are isomorphic. In a field of order pk, adding p copies of any element always results in zero; that is, the characteristic of the field is p.

In a finite field of order q, the polynomial XqX has all q elements of the finite field as roots. The non-zero elements of a finite field form a multiplicative group. This group is cyclic, so all non-zero elements can be expressed as powers of a single element called a primitive element of the field (in general there will be several primitive elements for a given field.)

A finite field is a finite set on which the four operations multiplication, addition, subtraction and division (excluding division by zero) are defined, satisfying the rules of arithmetic known as the field axioms. The simplest examples of finite fields are the fields of prime order: for each prime number p, the field GF(p) (also denoted Z/pZ, ${\displaystyle \mathbb {F} _{p}}$, or Fp) of order (that is, size) p may be constructed as the p. A more general algebraic structure that satisfies all the other axioms of a field, but whose multiplication is not required to be commutative, is called a division ring (or sometimes skewfield), however by Wedderburn's little theorem, any finite division ring must be commutative, and hence a finite field.

The elements of such a field may be represented by integers in the range 0, ..., p − 1. The sum, the difference and the product are computed by taking the remainder by p of the integer result. The multiplicative inverse of an element may be computed by using the extended Euclidean algorithm (see Extended Euclidean algorithm § Modular integers).

Let F be a finite field. For any element x in F and any integer n, let us denote by nx the sum of n copies of x. The least positive n such that n ⋅ 1 = 0 must exist and is a prime number; it is called the characteristic of the field.

If the characteristic of F is p, one can define multiplication of an element k of GF(p) by an element x of F ${\displaystyle (k,x)\mapsto k\cdot x}$ by choosing an integer representative for k and using repeated addition. This multiplication makes F into a GF(p)- vector space. It follows that the number of elements of F is pn for some integer n.

For every prime number p and every positive integer n, there exist finite fields of order pn, and all fields of this order are isomorphic (see § Existence and uniqueness below). One may therefore identify all fields of order pn, which are therefore unambiguously denoted ${\displaystyle \mathbb {F} _{p^{n}}}$, Fpn or GF(pn), where the letters GF stand for "Galois field". [1]

The identity

${\displaystyle (x+y)^{p}=x^{p}+y^{p}}$

(sometimes called the freshman's dream) is true (for every x and y) in a field of characteristic p. (This follows from each binomial coefficient of the expansion of (x + y)p, except the first and the last, being a multiple of p).

For each element x in the field GF(p) for a prime number p, one has xp = x (This is an immediate consequence of Fermat's little theorem, and this may be proved as follows: the equality is trivially true for x = 0 and x = 1; one obtains the result for the other elements of GF(p) by applying freshman's dreams with y = 1 and x taking successively the values 1, 2, ..., p − 1 modulo p.) This implies the equality

${\displaystyle X^{p}-X=\prod _{a\in {\rm {GF}}(p)}(X-a)}$

for polynomials over GF(p). More generally, every element in GF(pn) satisfies the polynomial equation xpnx = 0.

Any finite field extension of a finite field is separable and simple. That is, if E is a finite field and F is a subfield of E, then E is obtained from F by adjoining a single element whose minimal polynomial is separable. To use a jargon, finite fields are perfect.

Other Languages
العربية: حقل منته
беларуская: Канечнае поле
български: Крайно поле
català: Cos finit
español: Cuerpo finito
français: Corps fini
한국어: 유한체
italiano: Campo finito
עברית: שדה סופי

português: Corpo finito
română: Corp finit
Simple English: Galois field
српски / srpski: Коначно поље
svenska: Ändlig kropp
Türkçe: Sonlu alan
українська: Поле Галуа