Finite field

Inmathematics,afinite fieldorGalois field(so-named in honor ofÉvariste Galois) is afieldthat contains a finite number ofelements.As with any field, a finite field is aseton 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 theintegers mod $p$ when $p$ is aprime number.

Theorderof a finite field is its number of elements, which is either a prime number or aprime power.For every prime number $p$ and every positive integer $k$ there are fields of order $p k$ ,all of which areisomorphic.

Finite fields are fundamental in a number of areas of mathematics andcomputer science,includingnumber theory,algebraic geometry,Galois theory,finite geometry,cryptographyandcoding theory.

Properties[edit]

A finite field is a finite set that is afield;this means that multiplication, addition, subtraction and division (excluding division by zero) are defined and satisfy the rules of arithmetic known as thefield axioms.

The number of elements of a finite field is called itsorderor, sometimes, itssize.A finite field of order $q$ exists if and only if $q$ is aprime power $p k$ (where $p$ is a prime number and $k$ is a positive integer). In a field of order $p k$ ,adding $p$ copies of any element always results in zero; that is, thecharacteristicof the field is $p$ .

If $q = p k$ ,all fields of order $q$ areisomorphic(see§ Existence and uniquenessbelow).^[1]Moreover, a field cannot contain two different finitesubfieldswith the same order. One may therefore identify all finite fields with the same order, and they are unambiguously denoted $\mathbb {F} _{q}$ , $F q$ or $GF(q)$ ,where the letters GF stand for "Galois field".^[2]

In a finite field of order $q$ ,thepolynomial $X q - X$ has all $q$ elements of the finite field asroots.The non-zero elements of a finite field form amultiplicative group.This group iscyclic,so all non-zero elements can be expressed as powers of a single element called aprimitive elementof the field. (In general there will be several primitive elements for a given field.)

The simplest examples of finite fields are the fields of prime order: for eachprime number $p$ ,theprime fieldof order $p$ may be constructed as theintegers modulo $p$ , $\mathbb {Z} /p\mathbb {Z}$ .

The elements of the prime field of order $p$ may be represented by integers in the range $0,..., p - 1$ .The sum, the difference and the product are theremainder of the divisionby $p$ of the result of the corresponding integer operation. The multiplicative inverse of an element may be computed by using the extended Euclidean algorithm (seeExtended Euclidean algorithm § Modular integers).

Let $F$ be a finite field. For any element $x$ in $F$ and anyinteger $n$ ,denote by $n \cdot x$ the sum of $n$ copies of $x$ .The least positive $n$ such that $n \cdot 1 = 0$ is the characteristic $p$ of the field. This allows defining a multiplication $(k, x) \mapsto k \cdot x$ of an element $k$ of $GF(p)$ by an element $x$ of $F$ by choosing an integer representative for $k$ .This multiplication makes $F$ into a $GF(p)$ -vector space.It follows that the number of elements of $F$ is $p n$ for some integer $n$ .

Theidentity $(x+y)^{p}=x^{p}+y^{p}$ (sometimes called thefreshman's dream) is true in a field of characteristic $p$ .This follows from thebinomial theorem,as eachbinomial coefficientof the expansion of $(x + y) p$ ,except the first and the last, is a multiple of $p$ .

ByFermat's little theorem,if $p$ is a prime number and $x$ is in the field $GF(p)$ then $x p = x$ .This implies the equality $X^{p}-X=\prod _{a\in \mathrm {GF} (p)}(X-a)$ for polynomials over $GF(p)$ .More generally, every element in $GF(p n)$ satisfies the polynomial equation $x p n - x = 0$ .

Any finitefield extensionof a finite field isseparableand 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 whoseminimal polynomialisseparable.To use a piece of jargon, finite fields areperfect.

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 adivision ring(or sometimesskew field). ByWedderburn's little theorem,any finite division ring is commutative, and hence is a finite field.

Existence and uniqueness[edit]

Let $q = p n$ be aprime power,and $F$ be thesplitting fieldof the polynomial $P=X^{q}-X$ over the prime field $GF(p)$ .This means that $F$ is a finite field of lowest order, in which $P$ has $q$ distinct roots (theformal derivativeof $P$ is $P' = -1$ ,implying that $gcd(P, P') = 1$ ,which in general implies that the splitting field is aseparable extensionof the original). Theabove identityshows that the sum and the product of two roots of $P$ are roots of $P$ ,as well as the multiplicative inverse of a root of $P$ .In other words, the roots of $P$ form a field of order $q$ ,which is equal to $F$ by the minimality of the splitting field.

The uniqueness up to isomorphism of splitting fields implies thus that all fields of order $q$ are isomorphic. Also, if a field $F$ has a field of order $q = p k$ as a subfield, its elements are the $q$ roots of $X q - X$ ,and $F$ cannot contain another subfield of order $q$ .

In summary, we have the following classification theorem first proved in 1893 byE. H. Moore:^[1]

The order of a finite field is a prime power. For every prime power $q$ there are fields of order $q$ ,and they are all isomorphic. In these fields, every element satisfies
$x^{q}=x,$ and the polynomial $X q - X$ factors as
$X^{q}-X=\prod _{a\in F}(X-a).$

It follows that $GF(p n)$ contains a subfield isomorphic to $GF(p m)$ if and only if $m$ is a divisor of $n$ ;in that case, this subfield is unique. In fact, the polynomial $X p m - X$ divides $X p n - X$ if and only if $m$ is a divisor of $n$ .

Explicit construction[edit]

Non-prime fields[edit]

Given a prime power $q = p n$ with $p$ prime and $n > 1$ ,the field $GF(q)$ may be explicitly constructed in the following way. One first chooses anirreducible polynomial $P$ in $GF(p)[X]$ of degree $n$ (such an irreducible polynomial always exists). Then thequotient ring $\mathrm {GF} (q)=\mathrm {GF} (p)[X]/(P)$ of the polynomial ring $GF(p)[X]$ by the ideal generated by $P$ is a field of order $q$ .

More explicitly, the elements of $GF(q)$ are the polynomials over $GF(p)$ whose degree is strictly less than $n$ .The addition and the subtraction are those of polynomials over $GF(p)$ .The product of two elements is the remainder of theEuclidean divisionby $P$ of the product in $GF(p)[X]$ . The multiplicative inverse of a non-zero element may be computed with the extended Euclidean algorithm; seeExtended Euclidean algorithm § Simple algebraic field extensions.

However, with this representation, elements of $GF(q)$ may be difficult to distinguish from the corresponding polynomials. Therefore, it is common to give a name, commonly $α$ to the element of $GF(q)$ that corresponds to the polynomial $X$ .So, the elements of $GF(q)$ become polynomials in $α$ ,where $P (α) = 0$ ,and, when one encounters a polynomial in $α$ of degree greater or equal to $n$ (for example after a multiplication), one knows that one has to use the relation $P (α) = 0$ to reduce its degree (it is what Euclidean division is doing).

Except in the construction of $GF(4)$ ,there are several possible choices for $P$ ,which produce isomorphic results. To simplify the Euclidean division, one commonly chooses for $P$ a polynomial of the form $X^{n}+aX+b,$ which make the needed Euclidean divisions very efficient. However, for some fields, typically in characteristic $2$ ,irreducible polynomials of the form $X n + aX + b$ may not exist. In characteristic $2$ ,if the polynomial $X n + X + 1$ is reducible, it is recommended to choose $X n + X k + 1$ with the lowest possible $k$ that makes the polynomial irreducible. If all thesetrinomialsare reducible, one chooses "pentanomials" $X n + X a + X b + X c + 1$ ,as polynomials of degree greater than $1$ ,with an even number of terms, are never irreducible in characteristic $2$ ,having $1$ as a root.^[3]

A possible choice for such a polynomial is given byConway polynomials.They ensure a certain compatibility between the representation of a field and the representations of its subfields.

In the next sections, we will show how the general construction method outlined above works for small finite fields.

Field with four elements[edit]

The smallest non-prime field is the field with four elements, which is commonly denoted $GF(4)$ or $\mathbb {F} _{4}.$ It consists of the four elements $0, 1, α,1 + α$ such that $α 2 = 1 + α$ , $1 \cdot α = α \cdot 1 = α$ , $x + x = 0$ ,and $x \cdot 0 = 0 \cdot x = 0$ ,for every $x \in GF(4)$ ,the other operation results being easily deduced from thedistributive law.See below for the complete operation tables.

This may be deduced as follows from the results of the preceding section.

Over $GF(2)$ ,there is only oneirreducible polynomialof degree $2$ : $X^{2}+X+1$ Therefore, for $GF(4)$ the construction of the preceding section must involve this polynomial, and $\mathrm {GF} (4)=\mathrm {GF} (2)[X]/(X^{2}+X+1).$

Let $α$ denote a root of this polynomial in $GF(4)$ .This implies that

α 2 = 1 + α

,

and that $α$ and $1 + α$ are the elements of $GF(4)$ that are not in $GF(2)$ .The tables of the operations in $GF(4)$ result from this, and are as follows:

Addition

x + y

Multiplication

x \cdot y

Division

x / y

$y$ $x$	$0$	$1$	$α$	$1 + α$
$0$	$0$	$1$	$α$	$1 + α$
$1$	$1$	$0$	$1 + α$	$α$
$α$	$α$	$1 + α$	$0$	$1$
$1 + α$	$1 + α$	$α$	$1$	$0$

$y$ $x$	$0$	$1$	$α$	$1 + α$
$0$	$0$	$0$	$0$	$0$
$1$	$0$	$1$	$α$	$1 + α$
$α$	$0$	$α$	$1 + α$	$1$
$1 + α$	$0$	$1 + α$	$1$	$α$

$y$ $x$	$1$	$α$	$1 + α$
$0$	$0$	$0$	$0$
$1$	$1$	$1 + α$	$α$
$α$	$α$	$1$	$1 + α$
$1 + α$	$1 + α$	$α$	$1$

A table for subtraction is not given, because subtraction is identical to addition, as is the case for every field of characteristic 2. In the third table, for the division of $x$ by $y$ ,the values of $x$ must be read in the left column, and the values of $y$ in the top row. (Because $0 \cdot z = 0$ for every $z$ in everyringthedivision by 0has to remain undefined.) From the tables, it can be seen that the additive structure of $GF(4)$ is isomorphic to theKlein four-group,while the non-zero multiplicative structure is isomorphic to the group Z₃.

The map $\varphi:x\mapsto x^{2}$ is the non-trivial field automorphism, called theFrobenius automorphism,which sends $α$ into the second root $1 + α$ of the above mentioned irreducible polynomial $X 2 + X + 1$ .

GF(p²) for an odd primep[edit]

For applying theabove general constructionof finite fields in the case of $GF(p 2)$ ,one has to find an irreducible polynomial of degree 2. For $p = 2$ ,this has been done in the preceding section. If $p$ is an odd prime, there are always irreducible polynomials of the form $X 2 - r$ ,with $r$ in $GF(p)$ .

More precisely, the polynomial $X 2 - r$ is irreducible over $GF(p)$ if and only if $r$ is aquadratic non-residuemodulo $p$ (this is almost the definition of a quadratic non-residue). There are $.mw-parser-output .sfrac{white-space:nowrap}.mw-parser-output .sfrac.tion,.mw-parser-output .sfrac .tion{display:inline-block;vertical-align:-0.5em;font-size:85%;text-align:center}.mw-parser-output .sfrac .num{display:block;line-height:1em;margin:0.0em 0.1em;border-bottom:1px solid}.mw-parser-output .sfrac .den{display:block;line-height:1em;margin:0.1em 0.1em}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}p− 1/2$ quadratic non-residues modulo $p$ .For example, $2$ is a quadratic non-residue for $p = 3, 5, 11, 13,...$ ,and $3$ is a quadratic non-residue for $p = 5, 7, 17,...$ .If $p \equiv 3 mod 4$ ,that is $p = 3, 7, 11, 19,...$ ,one may choose $-1 \equiv p - 1$ as a quadratic non-residue, which allows us to have a very simple irreducible polynomial $X 2 + 1$ .

Having chosen a quadratic non-residue $r$ ,let $α$ be a symbolic square root of $r$ ,that is, a symbol that has the property $α 2 = r$ ,in the same way that the complex number $i$ is a symbolic square root of $-1$ .Then, the elements of $GF(p 2)$ are all the linear expressions $a+b\alpha,$ with $a$ and $b$ in $GF(p)$ .The operations on $GF(p 2)$ are defined as follows (the operations between elements of $GF(p)$ represented by Latin letters are the operations in $GF(p)$ ): ${\begin{aligned}-(a+b\alpha )&=-a+(-b)\alpha \\(a+b\alpha )+(c+d\alpha )&=(a+c)+(b+d)\alpha \\(a+b\alpha )(c+d\alpha )&=(ac+rbd)+(ad+bc)\alpha \\(a+b\alpha )^{-1}&=a(a^{2}-rb^{2})^{-1}+(-b)(a^{2}-rb^{2})^{-1}\alpha \end{aligned}}$

GF(8) and GF(27)[edit]

The polynomial $X^{3}-X-1$ is irreducible over $GF(2)$ and $GF(3)$ ,that is, it is irreducible modulo $2$ and $3$ (to show this, it suffices to show that it has no root in $GF(2)$ nor in $GF(3)$ ). It follows that the elements of $GF(8)$ and $GF(27)$ may be represented byexpressions $a+b\alpha +c\alpha ^{2},$ where $a, b, c$ are elements of $GF(2)$ or $GF(3)$ (respectively), and $α$ is a symbol such that $\alpha ^{3}=\alpha +1.$

The addition, additive inverse and multiplication on $GF(8)$ and $GF(27)$ may thus be defined as follows; in following formulas, the operations between elements of $GF(2)$ or $GF(3)$ ,represented by Latin letters, are the operations in $GF(2)$ or $GF(3)$ ,respectively: ${\begin{aligned}-(a+b\alpha +c\alpha ^{2})&=-a+(-b)\alpha +(-c)\alpha ^{2}\qquad {\text{(for }}\mathrm {GF} (8),{\text{this operation is the identity)}}\\(a+b\alpha +c\alpha ^{2})+(d+e\alpha +f\alpha ^{2})&=(a+d)+(b+e)\alpha +(c+f)\alpha ^{2}\\(a+b\alpha +c\alpha ^{2})(d+e\alpha +f\alpha ^{2})&=(ad+bf+ce)+(ae+bd+bf+ce+cf)\alpha +(af+be+cd+cf)\alpha ^{2}\end{aligned}}$

GF(16)[edit]

The polynomial $X^{4}+X+1$ is irreducible over $GF(2)$ ,that is, it is irreducible modulo $2$ .It follows that the elements of $GF(16)$ may be represented byexpressions $a+b\alpha +c\alpha ^{2}+d\alpha ^{3},$ where $a, b, c, d$ are either $0$ or $1$ (elements of $GF(2)$ ), and $α$ is a symbol such that $\alpha ^{4}=\alpha +1$ (that is, $α$ is defined as a root of the given irreducible polynomial). As the characteristic of $GF(2)$ is $2$ ,each element is its additive inverse in $GF(16)$ .The addition and multiplication on $GF(16)$ may be defined as follows; in following formulas, the operations between elements of $GF(2)$ ,represented by Latin letters are the operations in $GF(2)$ . ${\begin{aligned}(a+b\alpha +c\alpha ^{2}+d\alpha ^{3})+(e+f\alpha +g\alpha ^{2}+h\alpha ^{3})&=(a+e)+(b+f)\alpha +(c+g)\alpha ^{2}+(d+h)\alpha ^{3}\\(a+b\alpha +c\alpha ^{2}+d\alpha ^{3})(e+f\alpha +g\alpha ^{2}+h\alpha ^{3})&=(ae+bh+cg+df)+(af+be+bh+cg+df+ch+dg)\alpha \;+\\&\quad \;(ag+bf+ce+ch+dg+dh)\alpha ^{2}+(ah+bg+cf+de+dh)\alpha ^{3}\end{aligned}}$

The field $GF(16)$ has eightprimitive elements(the elements that have all nonzero elements of $GF(16)$ as integer powers). These elements are the four roots of $X 4 + X + 1$ and theirmultiplicative inverses.In particular, $α$ is a primitive element, and the primitive elements are $α m$ with $m$ less than and coprime with $15$ (that is, 1, 2, 4, 7, 8, 11, 13, 14).

Multiplicative structure[edit]

The set of non-zero elements in $GF(q)$ is anabelian groupunder the multiplication, of order $q - 1$ .ByLagrange's theorem,there exists a divisor $k$ of $q - 1$ such that $x k = 1$ for every non-zero $x$ in $GF(q)$ .As the equation $x k = 1$ has at most $k$ solutions in any field, $q - 1$ is the lowest possible value for $k$ . Thestructure theorem of finite abelian groupsimplies that this multiplicative group iscyclic,that is, all non-zero elements are powers of a single element. In summary:

The multiplicative group of the non-zero elements in

GF(q)

is cyclic, i.e., there exists an element

a

,such that the

q - 1

non-zero elements of

GF(q)

are

a, a 2,..., a q -2, a q -1 = 1

.

Such an element $a$ is called aprimitive elementof $GF(q)$ .Unless $q = 2, 3$ ,the primitive element is not unique. The number of primitive elements is $φ (q - 1)$ where $φ$ isEuler's totient function.

The result above implies that $x q = x$ for every $x$ in $GF(q)$ .The particular case where $q$ is prime isFermat's little theorem.

Discrete logarithm[edit]

If $a$ is a primitive element in $GF(q)$ ,then for any non-zero element $x$ in $F$ ,there is a unique integer $n$ with $0 \leq n \leq q - 2$ such that

x = a n

.

This integer $n$ is called thediscrete logarithmof $x$ to the base $a$ .

While $a n$ can be computed very quickly, for example usingexponentiation by squaring,there is no known efficient algorithm for computing the inverse operation, the discrete logarithm. This has been used in variouscryptographic protocols,seeDiscrete logarithmfor details.

When the nonzero elements of $GF(q)$ are represented by their discrete logarithms, multiplication and division are easy, as they reduce to addition and subtraction modulo $q - 1$ .However, addition amounts to computing the discrete logarithm of $a m + a n$ .The identity

a m + a n = a n (a m - n + 1)

allows one to solve this problem by constructing the table of the discrete logarithms of $a n + 1$ ,calledZech's logarithms,for $n = 0,..., q - 2$ (it is convenient to define the discrete logarithm of zero as being $-\infty$ ).

Zech's logarithms are useful for large computations, such aslinear algebraover medium-sized fields, that is, fields that are sufficiently large for making natural algorithms inefficient, but not too large, as one has to pre-compute a table of the same size as the order of the field.

Roots of unity[edit]

Every nonzero element of a finite field is aroot of unity,as $x q -1 = 1$ for every nonzero element of $GF(q)$ .

If $n$ is a positive integer, an $n$ thprimitive root of unityis a solution of the equation $x n = 1$ that is not a solution of the equation $x m = 1$ for any positive integer $m < n$ .If $a$ is a $n$ th primitive root of unity in a field $F$ ,then $F$ contains all the $n$ roots of unity, which are $1, a, a 2,..., a n -1$ .

The field $GF(q)$ contains a $n$ th primitive root of unity if and only if $n$ is a divisor of $q - 1$ ;if $n$ is a divisor of $q - 1$ ,then the number of primitive $n$ th roots of unity in $GF(q)$ is $φ (n)$ (Euler's totient function). The number of $n$ th roots of unity in $GF(q)$ is $gcd(n, q - 1)$ .

In a field of characteristic $p$ ,every $(np)$ th root of unity is also a $n$ th root of unity. It follows that primitive $(np)$ th roots of unity never exist in a field of characteristic $p$ .

On the other hand, if $n$ iscoprimeto $p$ ,the roots of the $n$ thcyclotomic polynomialare distinct in every field of characteristic $p$ ,as this polynomial is a divisor of $X n - 1$ ,whosediscriminant $n n$ is nonzero modulo $p$ .It follows that the $n$ thcyclotomic polynomialfactors over $GF(p)$ into distinct irreducible polynomials that have all the same degree, say $d$ ,and that $GF(p d)$ is the smallest field of characteristic $p$ that contains the $n$ th primitive roots of unity.

Example: GF(64)[edit]

The field $GF(64)$ has several interesting properties that smaller fields do not share: it has two subfields such that neither is contained in the other; not all generators (elements withminimal polynomialof degree $6$ over $GF(2)$ ) are primitive elements; and the primitive elements are not all conjugate under theGalois group.

The order of this field being $26$ ,and the divisors of $6$ being $1, 2, 3, 6$ ,the subfields of $GF(64)$ are $GF(2)$ , $GF(2 2) = GF(4)$ , $GF(2 3) = GF(8)$ ,and $GF(64)$ itself. As $2$ and $3$ arecoprime,the intersection of $GF(4)$ and $GF(8)$ in $GF(64)$ is the prime field $GF(2)$ .

The union of $GF(4)$ and $GF(8)$ has thus $10$ elements. The remaining $54$ elements of $GF(64)$ generate $GF(64)$ in the sense that no other subfield contains any of them. It follows that they are roots of irreducible polynomials of degree $6$ over $GF(2)$ .This implies that, over $GF(2)$ ,there are exactly $9 = 54 / 6$ irreduciblemonic polynomialsof degree $6$ .This may be verified by factoring $X 64 - X$ over $GF(2)$ .

The elements of $GF(64)$ are primitive $n$ th roots of unity for some $n$ dividing $63$ .As the 3rd and the 7th roots of unity belong to $GF(4)$ and $GF(8)$ ,respectively, the $54$ generators are primitive $n$ th roots of unity for some $n$ in ${9, 21, 63}$ .Euler's totient functionshows that there are $6$ primitive $9$ th roots of unity, $12$ primitive $21$ st roots of unity, and $36$ primitive $63$ rd roots of unity. Summing these numbers, one finds again $54$ elements.

By factoring thecyclotomic polynomialsover $GF(2)$ ,one finds that:

The six primitive $9$ th roots of unity are roots of $X^{6}+X^{3}+1,$ and are all conjugate under the action of the Galois group.
The twelve primitive $21$ st roots of unity are roots of $(X^{6}+X^{4}+X^{2}+X+1)(X^{6}+X^{5}+X^{4}+X^{2}+1).$ They form two orbits under the action of the Galois group. As the two factors arereciprocalto each other, a root and its (multiplicative) inverse do not belong to the same orbit.
The $36$ primitive elements of $GF(64)$ are the roots of $(X^{6}+X^{4}+X^{3}+X+1)(X^{6}+X+1)(X^{6}+X^{5}+1)(X^{6}+X^{5}+X^{3}+X^{2}+1)(X^{6}+X^{5}+X^{2}+X+1)(X^{6}+X^{5}+X^{4}+X+1).$ They split into six orbits of six elements each under the action of the Galois group.

This shows that the best choice to construct $GF(64)$ is to define it as $GF(2)[X] / (X 6 + X + 1)$ .In fact, this generator is a primitive element, and this polynomial is the irreducible polynomial that produces the easiest Euclidean division.

Frobenius automorphism and Galois theory[edit]

In this section, $p$ is a prime number, and $q = p n$ is a power of $p$ .

In $GF(q)$ ,the identity $(x + y) p = x p + y p$ implies that the map $\varphi:x\mapsto x^{p}$ is a $GF(p)$ -linear endomorphismand afield automorphismof $GF(q)$ ,which fixes every element of the subfield $GF(p)$ .It is called theFrobenius automorphism,afterFerdinand Georg Frobenius.

Denoting by $φ k$ thecompositionof $φ$ with itself $k$ times, we have $\varphi ^{k}:x\mapsto x^{p^{k}}.$ It has been shown in the preceding section that $φ n$ is the identity. For $0 < k < n$ ,the automorphism $φ k$ is not the identity, as, otherwise, the polynomial $X^{p^{k}}-X$ would have more than $p k$ roots.

There are no other $GF(p)$ -automorphisms of $GF(q)$ .In other words, $GF(p n)$ has exactly $n$ $GF(p)$ -automorphisms, which are $\mathrm {Id} =\varphi ^{0},\varphi,\varphi ^{2},\ldots,\varphi ^{n-1}.$

In terms ofGalois theory,this means that $GF(p n)$ is aGalois extensionof $GF(p)$ ,which has acyclicGalois group.

The fact that the Frobenius map is surjective implies that every finite field isperfect.

Polynomial factorization[edit]

If $F$ is a finite field, a non-constantmonic polynomialwith coefficients in $F$ isirreducibleover $F$ ,if it is not the product of two non-constant monic polynomials, with coefficients in $F$ .

As everypolynomial ringover a field is aunique factorization domain,every monic polynomial over a finite field may be factored in a unique way (up to the order of the factors) into a product of irreducible monic polynomials.

There are efficient algorithms for testing polynomial irreducibility and factoring polynomials over finite field. They are a key step for factoring polynomials over the integers or therational numbers.At least for this reason, everycomputer algebra systemhas functions for factoring polynomials over finite fields, or, at least, over finite prime fields.

Irreducible polynomials of a given degree[edit]

The polynomial $X^{q}-X$ factors into linear factors over a field of order $q$ .More precisely, this polynomial is the product of all monic polynomials of degree one over a field of order $q$ .

This implies that, if $q = p n$ then $X q - X$ is the product of all monic irreducible polynomials over $GF(p)$ ,whose degree divides $n$ .In fact, if $P$ is an irreducible factor over $GF(p)$ of $X q - X$ ,its degree divides $n$ ,as itssplitting fieldis contained in $GF(p n)$ .Conversely, if $P$ is an irreducible monic polynomial over $GF(p)$ of degree $d$ dividing $n$ ,it defines a field extension of degree $d$ ,which is contained in $GF(p n)$ ,and all roots of $P$ belong to $GF(p n)$ ,and are roots of $X q - X$ ;thus $P$ divides $X q - X$ .As $X q - X$ does not have any multiple factor, it is thus the product of all the irreducible monic polynomials that divide it.

This property is used to compute the product of the irreducible factors of each degree of polynomials over $GF(p)$ ;seeDistinct degree factorization.

Number of monic irreducible polynomials of a given degree over a finite field[edit]

The number $N (q, n)$ of monic irreducible polynomials of degree $n$ over $GF(q)$ is given by^[4] $N(q,n)={\frac {1}{n}}\sum _{d\mid n}\mu (d)q^{n/d},$ where $μ$ is theMöbius function.This formula is an immediate consequence of the property of $X q - X$ above and theMöbius inversion formula.

By the above formula, the number of irreducible (not necessarily monic) polynomials of degree $n$ over $GF(q)$ is $(q - 1) N (q, n)$ .

The exact formula implies the inequality $N(q,n)\geq {\frac {1}{n}}\left(q^{n}-\sum _{\ell \mid n,\ \ell {\text{ prime}}}q^{n/\ell }\right);$ this is sharp if and only if $n$ is a power of some prime. For every $q$ and every $n$ ,the right hand side is positive, so there is at least one irreducible polynomial of degree $n$ over $GF(q)$ .

Applications[edit]

Incryptography,the difficulty of thediscrete logarithm problemin finite fields or inelliptic curvesis the basis of several widely used protocols, such as theDiffie–Hellmanprotocol. For example, in 2014, a secure internet connection to Wikipedia involved the elliptic curve Diffie–Hellman protocol (ECDHE) over a large finite field.^[5]Incoding theory,many codes are constructed assubspacesofvector spacesover finite fields.

Finite fields are used by manyerror correction codes,such asReed–Solomon error correction codeorBCH code.The finite field almost always has characteristic of $2$ ,since computer data is stored in binary. For example, a byte of data can be interpreted as an element of $GF(2 8)$ .One exception isPDF417bar code, which is $GF(929)$ .Some CPUs have special instructions that can be useful for finite fields of characteristic $2$ ,generally variations ofcarry-less product.

Finite fields are widely used innumber theory,as many problems over the integers may be solved by reducing themmoduloone or severalprime numbers.For example, the fastest known algorithms forpolynomial factorizationandlinear algebraover the field ofrational numbersproceed by reduction modulo one or several primes, and then reconstruction of the solution by usingChinese remainder theorem,Hensel liftingor theLLL algorithm.

Similarly many theoretical problems in number theory can be solved by considering their reductions modulo some or all prime numbers. See, for example,Hasse principle.Many recent developments ofalgebraic geometrywere motivated by the need to enlarge the power of these modular methods.Wiles' proof of Fermat's Last Theoremis an example of a deep result involving many mathematical tools, including finite fields.

TheWeil conjecturesconcern the number of points onalgebraic varietiesover finite fields and the theory has many applications includingexponentialandcharacter sumestimates.

Finite fields have widespread application incombinatorics,two well known examples being the definition ofPaley Graphsand the related construction forHadamard Matrices.Inarithmetic combinatoricsfinite fields^[6]and finite field models^[7]^[8]are used extensively, such as inSzemerédi's theoremon arithmetic progressions.

Extensions[edit]

Wedderburn's little theorem[edit]

Adivision ringis a generalization of field. Division rings are not assumed to be commutative. There are no non-commutative finite division rings:Wedderburn's little theoremstates that all finitedivision ringsare commutative, and hence are finite fields. This result holds even if we relax theassociativityaxiom toalternativity,that is, all finitealternative division ringsare finite fields, by theArtin–Zorn theorem.^[9]

Algebraic closure[edit]

A finite field $F$ is not algebraically closed: the polynomial $f(T)=1+\prod _{\alpha \in F}(T-\alpha ),$ has no roots in $F$ ,since $f (α) = 1$ for all $α$ in $F$ .

Given a prime number $p$ ,let ${\overline {\mathbb {F} }}_{p}$ be an algebraic closure of $\mathbb {F} _{p}.$ It is not only uniqueup toan isomorphism, as do all algebraic closures, but contrarily to the general case, all its subfield are fixed by all its automorphisms, and it is also the algebraic closure of all finite fields of the same characteristic $p$ .

This property results mainly from the fact that the elements of $\mathbb {F} _{p^{n}}$ are exactly the roots of $x^{p^{n}}-x,$ and this defines an inclusion $\mathbb {\mathbb {F} } _{p^{n}}\subset \mathbb {F} _{p^{nm}}$ for $m>1.$ These inclusions allow writing informally ${\overline {\mathbb {F} }}_{p}=\bigcup _{n\geq 1}\mathbb {F} _{p^{n}}.$ The formal validation of this notation results from the fact that the above field inclusions form adirected setof fields; Itsdirect limitis ${\overline {\mathbb {F} }}_{p},$ which may thus be considered as "directed union".

Primitive elements in the algebraic closure[edit]

Given aprimitive element $g_{mn}$ of $\mathbb {F} _{q^{mn}},$ then $g_{mn}^{m}$ is a primitive element of $\mathbb {F} _{q^{n}}.$

For explicit computations, it may be useful to have a coherent choice of the primitive elements for all finite fields; that is, to choose the primitive element $g_{n}$ of $\mathbb {F} _{q^{n}}$ in order that, whenever $n=mh,$ one has $g_{m}=g_{n}^{h},$ where $g_{m}$ is the primitive element already chosen for $\mathbb {F} _{q^{m}}.$

Such a construction may be obtained byConway polynomials.

Quasi-algebraic closure[edit]

Although finite fields are not algebraically closed, they arequasi-algebraically closed,which means that everyhomogeneous polynomialover a finite field has a non-trivial zero whose components are in the field if the number of its variables is more than its degree. This was a conjecture ofArtinandDicksonproved byChevalley(seeChevalley–Warning theorem).

Notes[edit]

^^a ^bMoore, E. H.(1896), "A doubly-infinite system of simple groups", in E. H. Moore; et al. (eds.),Mathematical Papers Read at the International Mathematics Congress Held in Connection with the World's Columbian Exposition,Macmillan & Co., pp. 208–242
^This latter notation was introduced byE. H. Moorein an address given in 1893 at the International Mathematical Congress held in ChicagoMullen & Panario 2013,p. 10.
^Recommended Elliptic Curves for Government Use(PDF),National Institute of Standards and Technology,July 1999, p. 3,archived(PDF)from the original on 2008-07-19
^Jacobson 2009,§4.13
^This can be verified by looking at the information on the page provided by the browser.
^Shparlinski, Igor E. (2013), "Additive Combinatorics over Finite Fields: New Results and Applications",Finite Fields and Their Applications,DE GRUYTER, pp. 233–272,doi:10.1515/9783110283600.233,ISBN 9783110283600
^Green, Ben (2005), "Finite field models in additive combinatorics",Surveys in Combinatorics 2005,Cambridge University Press, pp. 1–28,arXiv:math/0409420,doi:10.1017/cbo9780511734885.002,ISBN 9780511734885,S2CID 28297089
^Wolf, J. (March 2015)."Finite field models in arithmetic combinatorics – ten years on".Finite Fields and Their Applications.32:233–274.doi:10.1016/j.ffa.2014.11.003.hdl:1983/d340f853-0584-49c8-a463-ea16ee51ce0f.ISSN 1071-5797.
^Shult, Ernest E. (2011).Points and lines. Characterizing the classical geometries.Universitext. Berlin:Springer-Verlag.p. 123.ISBN 978-3-642-15626-7.Zbl 1213.51001.

References[edit]

W. H. Bussey (1905) "Galois field tables forpⁿ≤ 169",Bulletin of the American Mathematical Society12(1): 22–38,doi:10.1090/S0002-9904-1905-01284-2
W. H. Bussey (1910) "Tables of Galois fields of order < 1000",Bulletin of the American Mathematical Society16(4): 188–206,doi:10.1090/S0002-9904-1910-01888-7
Jacobson, Nathan(2009) [1985],Basic algebra I(Second ed.), Dover Publications,ISBN 978-0-486-47189-1
Mullen, Gary L.; Mummert, Carl (2007),Finite Fields and Applications I,Student Mathematical Library (AMS),ISBN 978-0-8218-4418-2
Mullen, Gary L.; Panario, Daniel (2013),Handbook of Finite Fields,CRC Press,ISBN 978-1-4398-7378-6
Lidl, Rudolf;Niederreiter, Harald(1997),Finite Fields(2nd ed.),Cambridge University Press,ISBN 0-521-39231-4
Skopin, A. I. (2001) [1994],"Galois field",Encyclopedia of Mathematics,EMS Press

External links[edit]

Finite Fieldsat Wolfram research.

[moore-1] Moore, E. H.(1896), "A doubly-infinite system of simple groups", in E. H. Moore; et al. (eds.),Mathematical Papers Read at the International Mathematics Congress Held in Connection with the World's Columbian Exposition,Macmillan & Co., pp. 208–242

[2] This latter notation was introduced byE. H. Moorein an address given in 1893 at the International Mathematical Congress held in ChicagoMullen & Panario 2013,p. 10.

[3] Recommended Elliptic Curves for Government Use(PDF),National Institute of Standards and Technology,July 1999, p. 3,archived(PDF)from the original on 2008-07-19

[4] Jacobson 2009,§4.13

[5] This can be verified by looking at the information on the page provided by the browser.

[6] Shparlinski, Igor E. (2013), "Additive Combinatorics over Finite Fields: New Results and Applications",Finite Fields and Their Applications,DE GRUYTER, pp. 233–272,doi:10.1515/9783110283600.233,ISBN 9783110283600

[7] Green, Ben (2005), "Finite field models in additive combinatorics",Surveys in Combinatorics 2005,Cambridge University Press, pp. 1–28,arXiv:math/0409420,doi:10.1017/cbo9780511734885.002,ISBN 9780511734885,S2CID 28297089

[8] Wolf, J. (March 2015)."Finite field models in arithmetic combinatorics – ten years on".Finite Fields and Their Applications.32:233–274.doi:10.1016/j.ffa.2014.11.003.hdl:1983/d340f853-0584-49c8-a463-ea16ee51ce0f.ISSN 1071-5797.

[9] Shult, Ernest E. (2011).Points and lines. Characterizing the classical geometries.Universitext. Berlin:Springer-Verlag.p. 123.ISBN 978-3-642-15626-7.Zbl 1213.51001.

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