, a field is a set
on which addition
, and division
are defined and behave as the corresponding operations on rational
and real number
s do. A field is thus a fundamental algebraic structure
which is widely used in algebra
, number theory
, and many other areas of mathematics.
The best known fields are the field of rational number
s, the field of real number
s and the field of complex number
s. Many other fields, such as fields of rational functions
, algebraic function field
s, algebraic number field
s, and ''p''-adic fields
are commonly used and studied in mathematics, particularly in number theory and algebraic geometry
. Most cryptographic protocol
s rely on finite field
s, i.e., fields with finitely many elements
The relation of two fields is expressed by the notion of a field extension
. Galois theory
, initiated by Évariste Galois
in the 1830s, is devoted to understanding the symmetries of field extensions. Among other results, this theory shows that angle trisection
and squaring the circle
cannot be done with a compass and straightedge
. Moreover, it shows that quintic equation
s are, in general, algebraically unsolvable.
Fields serve as foundational notions in several mathematical domains. This includes different branches of mathematical analysis
, which are based on fields with additional structure. Basic theorems in analysis hinge on the structural properties of the field of real numbers. Most importantly for algebraic purposes, any field may be used as the scalars
for a vector space
, which is the standard general context for linear algebra
. Number field
s, the siblings of the field of rational numbers, are studied in depth in number theory
. Function fields
can help describe properties of geometric objects.
Informally, a field is a set, along with two operation
s defined on that set: an addition operation written as , and a multiplication operation written as , both of which behave similarly as they behave for rational number
s and real number
s, including the existence of an additive inverse
for all elements , and of a multiplicative inverse
for every nonzero element . This allows one to also consider the so-called ''inverse'' operations of subtraction
, , and division
, , by defining:
Formally, a field is a set
together with two binary operation
s on called ''addition'' and ''multiplication''. A binary operation on is a mapping , that is, a correspondence that associates with each ordered pair of elements of ''F'' a uniquely determined element of . The result of the addition of and is called the sum of and , and is denoted . Similarly, the result of the multiplication of and is called the product of and , and is denoted or . These operations are required to satisfy the following properties, referred to as ''field axioms
'' (in these axioms, , , and are arbitrary element
s of the field ):
of addition and multiplication: , and .
of addition and multiplication: , and .
and multiplicative identity
: there exist two different elements and in such that and .
* Additive inverse
s: for every in , there exists an element in , denoted , called the ''additive inverse'' of ''a'', such that .
* Multiplicative inverse
s: for every in , there exists an element in , denoted by or , called the ''multiplicative inverse'' of ''a'', such that .
of multiplication over addition: .
This may be summarized by saying: a field has two operations, called addition and multiplication; it is an abelian group
under addition with 0 as the additive identity; the nonzero elements are an abelian group under multiplication with 1 as the multiplicative identity; and multiplication distributes over addition.
Even more summarized: a field is a commutative ring
and all nonzero elements are invertible.
Fields can also be defined in different, but equivalent ways. One can alternatively define a field by four binary operations (addition, subtraction, multiplication, and division) and their required properties. Division by zero
is, by definition, excluded. In order to avoid existential quantifier
s, fields can be defined by two binary operations (addition and multiplication), two unary operations (yielding the additive and multiplicative inverses respectively), and two nullary
operations (the constants and ). These operations are then subject to the conditions above. Avoiding existential quantifiers is important in constructive mathematics
. One may equivalently define a field by the same two binary operations, one unary operation (the multiplicative inverse), and two constants and , since and .
[The a priori twofold use of the symbol "−" for denoting one part of a constant and for the additive inverses is justified by this latter condition.]
Rational numbers have been widely used a long time before the elaboration of the concept of field.
They are numbers that can be written as fractions
, where and are integer
s, and . The additive inverse of such a fraction is , and the multiplicative inverse (provided that ) is , which can be seen as follows:
The abstractly required field axioms reduce to standard properties of rational numbers. For example, the law of distributivity can be proven as follows:
Real and complex numbers
The real number
s , with the usual operations of addition and multiplication, also form a field. The complex number
s consist of expressions
: with real,
where is the imaginary unit
, i.e., a (non-real) number satisfying .
Addition and multiplication of real numbers are defined in such a way that expressions of this type satisfy all field axioms and thus hold for . For example, the distributive law enforces
It is immediate that this is again an expression of the above type, and so the complex numbers form a field. Complex numbers can be geometrically represented as points in the plane
, with Cartesian coordinates
given by the real numbers of their describing expression, or as the arrows from the origin to these points, specified by their length and an angle enclosed with some distinct direction. Addition then corresponds to combining the arrows to the intuitive parallelogram (adding the Cartesian coordinates), and the multiplication is – less intuitively – combining rotating and scaling of the arrows (adding the angles and multiplying the lengths). The fields of real and complex numbers are used throughout mathematics, physics, engineering, statistics, and many other scientific disciplines.
In antiquity, several geometric problems concerned the (in)feasibility of constructing certain numbers with compass and straightedge
. For example, it was unknown to the Greeks that it is, in general, impossible to trisect a given angle in this way. These problems can be settled using the field of constructible numbers
. Real constructible numbers are, by definition, lengths of line segments that can be constructed from the points 0 and 1 in finitely many steps using only compass
. These numbers, endowed with the field operations of real numbers, restricted to the constructible numbers, form a field, which properly includes the field of rational numbers. The illustration shows the construction of square root
s of constructible numbers, not necessarily contained within . Using the labeling in the illustration, construct the segments , , and a semicircle
over (center at the midpoint
), which intersects the perpendicular
line through in a point , at a distance of exactly
from when has length one.
Not all real numbers are constructible. It can be shown that
is not a constructible number, which implies that it is impossible to construct with compass and straightedge the length of the side of a cube with volume 2
, another problem posed by the ancient Greeks.
A field with four elements
In addition to familiar number systems such as the rationals, there are other, less immediate examples of fields. The following example is a field consisting of four elements called , , , and . The notation is chosen such that plays the role of the additive identity element (denoted 0 in the axioms above), and I is the multiplicative identity (denoted 1 in the axioms above). The field axioms can be verified by using some more field theory, or by direct computation. For example,
: , which equals , as required by the distributivity.
This field is called a finite field
with four elements, and is denoted or . The subset consisting of and (highlighted in red in the tables at the right) is also a field, known as the ''binary field
'' or . In the context of computer science
and Boolean algebra
, and are often denoted respectively by ''false'' and ''true'', the addition is then denoted XOR
(exclusive or), and the multiplication is denoted AND
. In other words, the structure of the binary field is the basic structure that allows computing with bit
In this section, denotes an arbitrary field and and are arbitrary elements
Consequences of the definition
One has and . In particular, one may deduce the additive inverse of every element as soon as one knows .
If then or must be 0, since, if , then
. This means that every field is an integral domain
In addition, the following properties are true for any elements and :
The additive and the multiplicative group of a field
The axioms of a field imply that it is an abelian group
under addition. This group is called the additive group
of the field, and is sometimes denoted by when denoting it simply as could be confusing.
Similarly, the ''nonzero'' elements of form an abelian group under multiplication, called the multiplicative group
, and denoted by or just or .
A field may thus be defined as set equipped with two operations denoted as an addition and a multiplication such that is an abelian group under addition, is an abelian group under multiplication (where 0 is the identity element of the addition), and multiplication is distributive
[Equivalently, a field is an algebraic structure of type , such that is not defined, and
are abelian groups, and
· is distributive over +. ]
Some elementary statements about fields can therefore be obtained by applying general facts of groups
. For example, the additive and multiplicative inverses and are uniquely determined by .
The requirement follows, because 1 is the identity element of a group that does not contain 0. Thus, the trivial ring
, consisting of a single element, is not a field.
Every finite subgroup of the multiplicative group of a field is cyclic
In addition to the multiplication of two elements of ''F'', it is possible to define the product of an arbitrary element of by a positive integer
to be the -fold sum
: (which is an element of .)
If there is no positive integer such that
then is said to have characteristic
0. For example, the field of rational numbers has characteristic 0 since no positive integer is zero. Otherwise, if there ''is'' a positive integer satisfying this equation, the smallest such positive integer can be shown to be a prime number
. It is usually denoted by and the field is said to have characteristic then.
For example, the field has characteristic 2 since (in the notation of the above addition table) .
If has characteristic , then for all in . This implies that
since all other binomial coefficient
s appearing in the binomial formula
are divisible by . Here, ( factors) is the -th power, i.e., the -fold product of the element . Therefore, the Frobenius map
is compatible with the addition in (and also with the multiplication), and is therefore a field homomorphism. The existence of this homomorphism makes fields in characteristic quite different from fields of characteristic 0.
Subfields and prime fields
'' of a field is a subset of that is a field with respect to the field operations of . Equivalently is a subset of that contains , and is closed under addition, multiplication, additive inverse and multiplicative inverse of a nonzero element. This means that , that for all both and are in , and that for all in , both and are in .
s are maps between two fields such that , , and , where and are arbitrary elements of . All field homomorphisms are injective
. If is also surjective
, it is called an isomorphism (or the fields and are called isomorphic).
A field is called a prime field
if it has no proper (i.e., strictly smaller) subfields. Any field contains a prime field. If the characteristic of is (a prime number), the prime field is isomorphic to the finite field introduced below. Otherwise the prime field is isomorphic to .
''Finite fields'' (also called ''Galois fields'') are fields with finitely many elements, whose number is also referred to as the order of the field. The above introductory example is a field with four elements. Its subfield is the smallest field, because by definition a field has at least two distinct elements .
The simplest finite fields, with prime order, are most directly accessible using modular arithmetic
. For a fixed positive integer , arithmetic "modulo " means to work with the numbers
The addition and multiplication on this set are done by performing the operation in question in the set of integers, dividing by and taking the remainder as result. This construction yields a field precisely if is a prime number
. For example, taking the prime results in the above-mentioned field . For and more generally, for any composite number
(i.e., any number which can be expressed as a product of two strictly smaller natural numbers), is not a field: the product of two non-zero elements is zero since in , which, as was explained above
, prevents from being a field. The field with elements ( being prime) constructed in this way is usually denoted by .
Every finite field has elements, where is prime and . This statement holds since may be viewed as a vector space
over its prime field. The dimension
of this vector space is necessarily finite, say , which implies the asserted statement.
A field with elements can be constructed as the splitting field
of the polynomial
Such a splitting field is an extension of in which the polynomial has zeros. This means has as many zeros as possible since the degree
of is . For , it can be checked case by case using the above multiplication table that all four elements of satisfy the equation , so they are zeros of . By contrast, in , has only two zeros (namely 0 and 1), so does not split into linear factors in this smaller field. Elaborating further on basic field-theoretic notions, it can be shown that two finite fields with the same order are isomorphic. It is thus customary to speak of ''the'' finite field with elements, denoted by or .
Historically, three algebraic disciplines led to the concept of a field: the question of solving polynomial equations, algebraic number theory
, and algebraic geometry
. A first step towards the notion of a field was made in 1770 by Joseph-Louis Lagrange
, who observed that permuting the zeros of a cubic polynomial
in the expression
(with being a third root of unity
) only yields two values. This way, Lagrange conceptually explained the classical solution method of Scipione del Ferro
and François Viète
, which proceeds by reducing a cubic equation for an unknown to a quadratic equation for . Together with a similar observation for equations of degree 4
, Lagrange thus linked what eventually became the concept of fields and the concept of groups. Vandermonde
, also in 1770, and to a fuller extent, Carl Friedrich Gauss
, in his ''Disquisitiones Arithmeticae
'' (1801), studied the equation
for a prime and, again using modern language, the resulting cyclic Galois group
. Gauss deduced that a regular -gon
can be constructed if . Building on Lagrange's work, Paolo Ruffini
claimed (1799) that quintic equation
s (polynomial equations of degree 5) cannot be solved algebraically; however, his arguments were flawed. These gaps were filled by Niels Henrik Abel
in 1824. Évariste Galois
, in 1832, devised necessary and sufficient criteria for a polynomial equation to be algebraically solvable, thus establishing in effect what is known as Galois theory
today. Both Abel and Galois worked with what is today called an algebraic number field
, but conceived neither an explicit notion of a field, nor of a group.
In 1871 Richard Dedekind
introduced, for a set of real or complex numbers that is closed under the four arithmetic operations, the German
word ''Körper'', which means "body" or "corpus" (to suggest an organically closed entity). The English term "field" was introduced by .
In 1881 Leopold Kronecker
defined what he called a ''domain of rationality'', which is a field of rational fraction
s in modern terms. Kronecker's notion did not cover the field of all algebraic numbers (which is a field in Dedekind's sense), but on the other hand was more abstract than Dedekind's in that it made no specific assumption on the nature of the elements of a field. Kronecker interpreted a field such as abstractly as the rational function field . Prior to this, examples of transcendental numbers were known since Joseph Liouville
's work in 1844, until Charles Hermite
(1873) and Ferdinand von Lindemann
(1882) proved the transcendence of and , respectively.
The first clear definition of an abstract field is due to . In particular, Heinrich Martin Weber
's notion included the field F''p''
. Giuseppe Veronese
(1891) studied the field of formal power series, which led to introduce the field of ''p''-adic numbers. synthesized the knowledge of abstract field theory accumulated so far. He axiomatically studied the properties of fields and defined many important field-theoretic concepts. The majority of the theorems mentioned in the sections Galois theory
, Constructing fields
and Elementary notions
can be found in Steinitz's work. linked the notion of orderings in a field
, and thus the area of analysis, to purely algebraic properties. Emil Artin
redeveloped Galois theory from 1928 through 1942, eliminating the dependency on the primitive element theorem
Constructing fields from rings
A commutative ring
is a set, equipped with an addition and multiplication operation, satisfying all the axioms of a field, except for the existence of multiplicative inverses . For example, the integers form a commutative ring, but not a field: the reciprocal
of an integer is not itself an integer, unless .
In the hierarchy of algebraic structures fields can be characterized as the commutative rings in which every nonzero element is a unit
(which means every element is invertible). Similarly, fields are the commutative rings with precisely two distinct ideal
s, and . Fields are also precisely the commutative rings in which is the only prime ideal
Given a commutative ring , there are two ways to construct a field related to , i.e., two ways of modifying such that all nonzero elements become invertible: forming the field of fractions, and forming residue fields. The field of fractions of is , the rationals, while the residue fields of are the finite fields .
Field of fractions
Given an integral domain
, its field of fractions
is built with the fractions of two elements of exactly as Q is constructed from the integers. More precisely, the elements of are the fractions where and are in , and . Two fractions and are equal if and only if . The operation on the fractions work exactly as for rational numbers. For example,
It is straightforward to show that, if the ring is an integral domain, the set of the fractions form a field.
The field of the rational fraction
s over a field (or an integral domain) is the field of fractions of the polynomial ring
. The field of Laurent series
over a field is the field of fractions of the ring of formal power series
(in which ). Since any Laurent series is a fraction of a power series divided by a power of (as opposed to an arbitrary power series), the representation of fractions is less important in this situation, though.
In addition to the field of fractions, which embeds injectively
into a field, a field can be obtained from a commutative ring by means of a surjective map
onto a field . Any field obtained in this way is a quotient
, where is a maximal ideal
of . If has only one maximal ideal
, this field is called the residue field
The ideal generated by a single polynomial
in the polynomial ring (over a field ) is maximal if and only if is irreducible
in , i.e., if cannot be expressed as the product of two polynomials in of smaller degree
. This yields a field
This field contains an element (namely the residue class
of ) which satisfies the equation
For example, is obtained from by adjoining
the imaginary unit
symbol , which satisfies , where . Moreover, is irreducible over , which implies that the map that sends a polynomial to yields an isomorphism
Constructing fields within a bigger field
Fields can be constructed inside a given bigger container field. Suppose given a field , and a field containing as a subfield. For any element of , there is a smallest subfield of containing and , called the subfield of ''F'' generated by and denoted . The passage from to is referred to by ''adjoining
an element'' to . More generally, for a subset , there is a minimal subfield of containing and , denoted by .
of two subfields and of some field is the smallest subfield of containing both and The compositum can be used to construct the biggest subfield of satisfying a certain property, for example the biggest subfield of , which is, in the language introduced below, algebraic over .
[Further examples include the maximal unramified extension or the maximal abelian extension within .]
The notion of a subfield can also be regarded from the opposite point of view, by referring to being a ''field extension
'' (or just extension) of , denoted by
and read " over ".
A basic datum of a field extension is its degree
, i.e., the dimension of as an -vector space. It satisfies the formula
Extensions whose degree is finite are referred to as finite extensions. The extensions and are of degree 2, whereas is an infinite extension.
A pivotal notion in the study of field extensions are algebraic element
s. An element is ''algebraic'' over if it is a root
of a polynomial
with coefficients in , that is, if it satisfies a polynomial equation
with in , and .
For example, the imaginary unit
in is algebraic over , and even over , since it satisfies the equation
A field extension in which every element of is algebraic over is called an algebraic extension
. Any finite extension is necessarily algebraic, as can be deduced from the above multiplicativity formula.
The subfield generated by an element , as above, is an algebraic extension of if and only if is an algebraic element. That is to say, if is algebraic, all other elements of are necessarily algebraic as well. Moreover, the degree of the extension , i.e., the dimension of as an -vector space, equals the minimal degree such that there is a polynomial equation involving , as above. If this degree is , then the elements of have the form
For example, the field of Gaussian rational
s is the subfield of consisting of all numbers of the form where both and are rational numbers: summands of the form (and similarly for higher exponents) don't have to be considered here, since can be simplified to .
The above-mentioned field of rational fraction
s , where is an indeterminate
, is not an algebraic extension of since there is no polynomial equation with coefficients in whose zero is . Elements, such as , which are not algebraic are called transcendental
. Informally speaking, the indeterminate and its powers do not interact with elements of . A similar construction can be carried out with a set of indeterminates, instead of just one.
Once again, the field extension discussed above is a key example: if is not algebraic (i.e., is not a root
of a polynomial with coefficients in ), then is isomorphic to . This isomorphism is obtained by substituting to in rational fractions.
A subset of a field is a transcendence basis
if it is algebraically independent
(don't satisfy any polynomial relations) over and if is an algebraic extension of . Any field extension has a transcendence basis. Thus, field extensions can be split into ones of the form (purely transcendental extensions
) and algebraic extensions.
A field is algebraically closed
if it does not have any strictly bigger algebraic extensions or, equivalently, if any polynomial equation
:, with coefficient
has a solution . By the fundamental theorem of algebra
, is algebraically closed, i.e., ''any'' polynomial equation with complex coefficients has a complex solution. The rational and the real numbers are ''not'' algebraically closed since the equation
does not have any rational or real solution. A field containing is called an ''algebraic closure
'' of if it is algebraic
over (roughly speaking, not too big compared to ) and is algebraically closed (big enough to contain solutions of all polynomial equations).
By the above, is an algebraic closure of . The situation that the algebraic closure is a finite extension of the field is quite special: by the Artin-Schreier theorem
, the degree of this extension is necessarily 2, and is elementarily equivalent
to . Such fields are also known as real closed field
Any field has an algebraic closure, which is moreover unique up to (non-unique) isomorphism. It is commonly referred to as ''the'' algebraic closure and denoted . For example, the algebraic closure of is called the field of algebraic number
s. The field is usually rather implicit since its construction requires the ultrafilter lemma
, a set-theoretic axiom that is weaker than the axiom of choice
. In this regard, the algebraic closure of , is exceptionally simple. It is the union of the finite fields containing (the ones of order ). For any algebraically closed field of characteristic 0, the algebraic closure of the field of Laurent series
is the field of Puiseux series
, obtained by adjoining roots of .
Fields with additional structure
Since fields are ubiquitous in mathematics and beyond, several refinements of the concept have been adapted to the needs of particular mathematical areas.
A field ''F'' is called an ''ordered field'' if any two elements can be compared, so that and whenever and . For example, the real numbers form an ordered field, with the usual ordering . The Artin-Schreier theorem
states that a field can be ordered if and only if it is a formally real field
, which means that any quadratic equation
only has the solution . The set of all possible orders on a fixed field is isomorphic to the set of ring homomorphism
s from the Witt ring
of quadratic form
s over , to .
An Archimedean field
is an ordered field such that for each element there exists a finite expression
whose value is greater than that element, that is, there are no infinite elements. Equivalently, the field contains no infinitesimals
(elements smaller than all rational numbers); or, yet equivalent, the field is isomorphic to a subfield of .
An ordered field is Dedekind-complete
if all upper bound
s, lower bound
s (see Dedekind cut
) and limits, which should exist, do exist. More formally, each bounded subset
of is required to have a least upper bound. Any complete field is necessarily Archimedean, since in any non-Archimedean field there is neither a greatest infinitesimal nor a least positive rational, whence the sequence , every element of which is greater than every infinitesimal, has no limit.
Since every proper subfield of the reals also contains such gaps, is the unique complete ordered field, up to isomorphism. Several foundational results in calculus
follow directly from this characterization of the reals.
form an ordered field that is not Archimedean. It is an extension of the reals obtained by including infinite and infinitesimal numbers. These are larger, respectively smaller than any real number. The hyperreals form the foundational basis of non-standard analysis
Another refinement of the notion of a field is a topological field
, in which the set is a topological space
, such that all operations of the field (addition, multiplication, the maps and ) are continuous map
s with respect to the topology of the space.
The topology of all the fields discussed below is induced from a metric
, i.e., a function
that measures a ''distance'' between any two elements of .
of is another field in which, informally speaking, the "gaps" in the original field are filled, if there are any. For example, any irrational number
, such as , is a "gap" in the rationals in the sense that it is a real number that can be approximated arbitrarily closely by rational numbers , in the sense that distance of and given by the absolute value
is as small as desired.
The following table lists some examples of this construction. The fourth column shows an example of a zero sequence
, i.e., a sequence whose limit (for ) is zero.
The field is used in number theory and -adic analysis
. The algebraic closure carries a unique norm extending the one on , but is not complete. The completion of this algebraic closure, however, is algebraically closed. Because of its rough analogy to the complex numbers, it is sometimes called the field of Metric completions and algebraic closures|complex p-adic number
s and is denoted by .
The following topological fields are called ''local field
[Some authors also consider the fields and to be local fields. On the other hand, these two fields, also called Archimedean local fields, share little similarity with the local fields considered here, to a point that calls them "completely anomalous".]
* finite extensions of (local fields of characteristic zero)
* finite extensions of , the field of Laurent series over (local fields of characteristic ).
These two types of local fields share some fundamental similarities. In this relation, the elements and (referred to as uniformizer
) correspond to each other. The first manifestation of this is at an elementary level: the elements of both fields can be expressed as power series in the uniformizer, with coefficients in . (However, since the addition in is done using carry
ing, which is not the case in , these fields are not isomorphic.) The following facts show that this superficial similarity goes much deeper:
* Any first order
statement that is true for almost all is also true for almost all . An application of this is the Ax-Kochen theorem
describing zeros of homogeneous polynomials in .
* Tamely ramified extension
s of both fields are in bijection to one another.
* Adjoining arbitrary -power roots of (in ), respectively of (in ), yields (infinite) extensions of these fields known as perfectoid field
s. Strikingly, the Galois groups of these two fields are isomorphic, which is the first glimpse of a remarkable parallel between these two fields:
s are fields equipped with a derivation
, i.e., allow to take derivatives of elements in the field. For example, the field R(''X''), together with the standard derivative of polynomials forms a differential field. These fields are central to differential Galois theory
, a variant of Galois theory dealing with linear differential equation
Galois theory studies algebraic extension
s of a field by studying the symmetry
in the arithmetic operations of addition and multiplication. An important notion in this area is that of finite Galois extension
s , which are, by definition, those that are separable
. The primitive element theorem
shows that finite separable extensions are necessarily simple
, i.e., of the form
where is an irreducible polynomial (as above). For such an extension, being normal and separable means that all zeros of are contained in and that has only simple zeros. The latter condition is always satisfied if has characteristic 0.
For a finite Galois extension, the Galois group
is the group of field automorphism
s of that are trivial on (i.e., the bijection
s that preserve addition and multiplication and that send elements of to themselves). The importance of this group stems from the fundamental theorem of Galois theory
, which constructs an explicit one-to-one correspondence
between the set of subgroup
s of and the set of intermediate extensions of the extension . By means of this correspondence, group-theoretic properties translate into facts about fields. For example, if the Galois group of a Galois extension as above is not solvable
(cannot be built from abelian group
s), then the zeros of ''cannot'' be expressed in terms of addition, multiplication, and radicals, i.e., expressions involving