GF(2)
View on WikipediaGF(2) (also denoted , Z/2Z or ) is the finite field with two elements.[1][a]
GF(2) is the field with the smallest possible number of elements, and is unique if the additive identity and the multiplicative identity are denoted respectively 0 and 1, as usual.
The elements of GF(2) may be identified with the two possible values of a bit and to the Boolean values true and false. It follows that GF(2) is fundamental and ubiquitous in computer science and its logical foundations.
Definition
[edit]GF(2) is the unique field with two elements with its additive and multiplicative identities respectively denoted 0 and 1.
Its addition is defined as the usual addition of integers but modulo 2 and corresponds to the table below:
| + | 0 | 1 |
|---|---|---|
| 0 | 0 | 1 |
| 1 | 1 | 0 |
If the elements of GF(2) are seen as Boolean values, then the addition is the same as that of the logical XOR operation. Since each element equals its opposite, subtraction is thus the same operation as addition.
The multiplication of GF(2) is the usual multiplication (see the table below), and on Boolean variables corresponds to the logical AND operation.
| × | 0 | 1 |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 0 | 1 |
GF(2) can be identified with the field of the integers modulo 2, that is, the quotient ring of the ring of integers Z by the ideal 2Z of all even numbers: GF(2) = Z/2Z.
Notations Z2 and may be encountered although they can be confused with the notation of 2-adic integers.
Properties
[edit]Because GF(2) is a field, many of the familiar properties of number systems such as the rational numbers and real numbers are retained:
- addition has an identity element (0) and an inverse for every element;
- multiplication has an identity element (1) and an inverse for every element but 0;
- addition and multiplication are commutative and associative;
- multiplication is distributive over addition.
Properties that are not familiar from the real numbers include:
- every element x of GF(2) satisfies x + x = 0 and therefore −x = x; this means that the characteristic of GF(2) is 2;
- every element x of GF(2) satisfies x2 = x (i.e. is idempotent with respect to multiplication); this is an instance of Fermat's little theorem. GF(2) is the only field with this property (Proof: if x2 = x, then either x = 0 or x ≠ 0. In the latter case, x must have a multiplicative inverse, in which case dividing both sides by x gives x = 1. All larger fields contain elements other than 0 and 1, and those elements cannot satisfy this property).
Applications
[edit]Because of the algebraic properties above, many familiar and powerful tools of mathematics work in GF(2) just as well as other fields. For example, matrix operations, including matrix inversion, can be applied to matrices with elements in GF(2) (see matrix ring).
Any group (V,+) with the property v + v = 0 for every v in V is necessarily abelian and can be turned into a vector space over GF(2) in a natural fashion, by defining 0v = 0 and 1v = v for all v in V. This vector space will have a basis, implying that the number of elements of V must be a power of 2 (or infinite).
In modern computers, data are represented with bit strings of a fixed length, called machine words. These are endowed with the structure of a vector space over GF(2). The addition of this vector space is the bitwise operation called XOR (exclusive or). The bitwise AND is another operation on this vector space, which makes it a Boolean algebra, a structure that underlies all computer science. These spaces can also be augmented with a multiplication operation that makes them into a field GF(2n), but the multiplication operation cannot be a bitwise operation. When n is itself a power of two, the multiplication operation can be nim-multiplication; alternatively, for any n, one can use multiplication of polynomials over GF(2) modulo a irreducible polynomial (as for instance for the field GF(28) in the description of the Advanced Encryption Standard cipher).
Vector spaces and polynomial rings over GF(2) are widely used in coding theory, and in particular in error correcting codes and modern cryptography. For example, many common error correcting codes (such as BCH codes) are linear codes over GF(2) (codes defined from vector spaces over GF(2)), or polynomial codes (codes defined as quotients of polynomial rings over GF(2)).
Algebraic closure
[edit]Like any field, GF(2) has an algebraic closure. This is a field F which contains GF(2) as a subfield, which is algebraic over GF(2) (i.e. every element of F is a root of a polynomial with coefficients in GF(2)), and which is algebraically closed (any non-constant polynomial with coefficients in F has a root in F). The field F is uniquely determined by these properties, up to a field automorphism (i.e. essentially up to the notation of its elements).
F is countable and contains a single copy of each of the finite fields GF(2n); the copy of GF(2n) is contained in the copy of GF(2m) if and only if n divides m. The field F is countable and is the union of all these finite fields.
Conway realized that F can be identified with the ordinal number , where the addition and multiplication operations are defined in a natural manner by transfinite induction (these operations are however different from the standard addition and multiplication of ordinal numbers).[2] The addition in this field is simple to perform and is akin to Nim-addition; Lenstra has shown that the multiplication can also be performed efficiently.[3]
See also
[edit]References
[edit]- ^ GF is the initialism of Galois field, another name for finite fields.
- ^ Lidl, Rudolf; Niederreiter, Harald (1997). Finite fields. Encyclopedia of Mathematics and Its Applications. Vol. 20 (2nd ed.). Cambridge University Press. ISBN 0-521-39231-4. Zbl 0866.11069.
- ^ Conway, John H. (2000). On Numbers and Games (2nd ed.). Wellesley, Mass. p. 61. ISBN 978-1-56881-127-7.
{{cite book}}: CS1 maint: location missing publisher (link) - ^ Lenstra, Hendrik (1977). "On the Algebraic Closure of Two" (PDF). Indagationes Mathematicae (Proceedings). 80 (5): 389–396. doi:10.1016/1385-7258(77)90053-1.
GF(2)
View on Grokipedia| + | 0 | 1 |
|---|---|---|
| 0 | 0 | 1 |
| 1 | 1 | 0 |
| × | 0 | 1 |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 0 | 1 |
Fundamentals
Definition
GF(2), also denoted , is the unique finite field of order 2, consisting of the elements [0](/page/0), 1\} with addition and multiplication performed modulo 2.[6] It serves as the prime field of characteristic 2, isomorphic to the quotient ring , where the integers are reduced modulo the prime 2.[6] Alternatively, GF(2) can be constructed as the splitting field of the polynomial over the prime field of characteristic 2, as both roots 0 and 1 lie within the field and satisfy for every element .[6] To verify that GF(2) is a field, it must satisfy the field axioms: it forms a commutative ring with unity under addition and multiplication, every non-zero element is invertible, and the distributive law holds. The additive group is cyclic of order 2, with 0 as the identity and each element its own inverse (since [0](/page/0)). The multiplicative structure has 1 as the unity, and the non-zero element 1 is its own inverse (). Commutativity, associativity, and distributivity follow directly from the corresponding properties in , verifiable by direct inspection of the two elements.[3][7] GF(2) is unique up to isomorphism: any field with exactly two elements must be isomorphic to it, as all finite fields of the same order (here , ) are isomorphic, determined by the existence and uniqueness of the splitting field of over the prime field .[6][8]Arithmetic Operations
In GF(2), the field with two elements denoted as {0, 1}, addition is defined as the operation modulo 2, which corresponds to the bitwise exclusive OR (XOR) for binary values. Specifically, the addition table yields: , , , and .[1][3] Subtraction in GF(2) coincides with addition due to the field's characteristic of 2, where each element is its own additive inverse, so for all .[9][10] Multiplication in GF(2) follows the standard binary rules, equivalent to the logical AND operation: , , , and .[1][3] The additive identity is 0, satisfying for all , while the multiplicative identity is 1, with . Additionally, the additive inverse of any element is itself, as [0](/page/0).[1][10] For division, since the only non-zero element is 1 and , the multiplicative inverse of 1 is 1 itself, making division by a non-zero element equivalent to multiplication by , so for [0](/page/0). Division by 0 is undefined, consistent with field axioms.[9]Properties
Characteristic and Structure
The field GF(2) has characteristic 2, defined as the smallest positive integer such that , where 1 denotes the multiplicative identity; this immediately implies that .[11][12] As the prime field of characteristic 2, GF(2) is the smallest field with this property, consisting of the elements {0, 1} and generated additively and multiplicatively by 1, with no proper subfields.[13][12] The multiplicative group of nonzero elements, denoted GF(2)^\times, is trivial, comprising solely the element 1 and thus having order 1; this follows from the field's order being 2, making the group cyclic of order .[11][13] As an integral domain (with no zero divisors, since every nonzero element has a multiplicative inverse), GF(2) exhibits further structural simplicity: every element is idempotent under multiplication, satisfying for , a property characteristic of Boolean rings.[14][15] This idempotence and the field's characteristic 2 align GF(2) closely with Boolean algebra, where the addition operation corresponds to the logical XOR gate and multiplication to the AND gate, enabling direct isomorphism between the field's structure and two-element Boolean lattices in computational contexts.[11][14]Vector Spaces over GF(2)
A vector space over GF(2), the finite field with two elements {0, 1}, consists of vectors whose components are elements of GF(2), with scalar multiplication and vector addition defined using the field's arithmetic operations modulo 2.[16] This binary nature means that any vector can be represented as a string of 0s and 1s, where addition corresponds to bitwise XOR and scalar multiplication by 1 leaves the vector unchanged while multiplication by 0 yields the zero vector.[17] Due to the field's characteristic 2, as discussed in prior sections on field properties, addition is idempotent (adding a vector to itself results in zero), which simplifies many structural aspects compared to vector spaces over fields of characteristic not equal to 2.[16] The dimension of a vector space over GF(2) is the cardinality of a basis, and for an n-dimensional space, the total number of vectors is .[17] A canonical example is , the set of all n-tuples with entries in GF(2), which forms an n-dimensional vector space under componentwise addition and scalar multiplication.[16] For instance, in , a basis is , and the four elements are , , , and , interpretable as 2-bit binary strings.[17] Every vector in this space is uniquely expressed as a linear combination of basis vectors with coefficients in {0,1}, reflecting the field's binary structure.[16] Linear independence in a vector space over GF(2) requires that no vector in the set is equal to the sum (via XOR) of the others.[17] A set of vectors is linearly dependent if there exist coefficients , not all zero, such that , where the sum is modulo 2; otherwise, the set is independent.[16] The maximum size of a linearly independent set equals the dimension, and bases provide minimal spanning sets for the space.[17] The standard inner product on is defined as for vectors and .[17] Two vectors are orthogonal if their inner product is 0, which plays a key role in concepts like orthogonal complements and self-orthogonal subspaces over GF(2).[16] This bilinear form is symmetric due to the commutativity of multiplication in GF(2). In characteristic 2, it satisfies , the parity of the Hamming weight of x, distinguishing it from positive definite inner products over the reals.[17]Representations
As a Quotient Ring
The field GF(2) is isomorphic to the quotient ring , obtained by factoring out the principal ideal generated by the prime number 2 from the ring of integers .[18] This construction yields a field because the ideal is maximal (equivalently, prime) in the principal ideal domain , ensuring that the quotient has no zero divisors and every nonzero element is invertible.[18] The elements of this quotient ring are the residue classes [0] = 2\mathbb{Z} and [1] = 1 + 2\mathbb{Z}, which are canonically identified with the set , where addition and multiplication are defined componentwise modulo 2.[18] As an integral domain of characteristic 2, GF(2) is already a field, so its field of fractions coincides with itself; there are no proper fractions beyond the elements already present, reflecting the finite nature of the structure.[19] This self-contained property underscores its role as the prime field of characteristic 2.[19] GF(2) can also be realized as a quotient of the polynomial ring by the ideal generated by 2 and a suitable polynomial that enforces the field's structure, such as , which collapses to by setting .[20] More generally, finite fields arise as quotients of polynomial rings over prime fields, but for GF(2) as the base case, the integer quotient provides the foundational construction.[20] The arithmetic underlying GF(2) was discovered implicitly through 19th-century developments in modular arithmetic, notably in Carl Friedrich Gauss's Disquisitiones Arithmeticae (1801), where congruences modulo primes laid the groundwork for finite structures, though the explicit field-theoretic interpretation emerged later with Évariste Galois's work around 1830.[21]Explicit Operation Tables
The operations in GF(2), the finite field with two elements {0, 1}, are defined modulo 2, making addition equivalent to the exclusive-or (XOR) operation and multiplication equivalent to the logical AND operation.[22] The addition table for GF(2) is as follows:| + | 0 | 1 |
|---|---|---|
| 0 | 0 | 1 |
| 1 | 1 | 0 |
| × | 0 | 1 |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 0 | 1 |