Английская Википедия:Glossary of field theory
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Шаблон:Short description Шаблон:Refimprove Field theory is the branch of mathematics in which fields are studied. This is a glossary of some terms of the subject. (See field theory (physics) for the unrelated field theories in physics.)
Definition of a field
A field is a commutative ring Шаблон:Nowrap in which Шаблон:Nowrap and every nonzero element has a multiplicative inverse. In a field we thus can perform the operations addition, subtraction, multiplication, and division.
The non-zero elements of a field F form an abelian group under multiplication; this group is typically denoted by F×;
The ring of polynomials in the variable x with coefficients in F is denoted by F[x].
Basic definitions
- Characteristic
- The characteristic of the field F is the smallest positive integer n such that Шаблон:Nowrap; here n·1 stands for n summands Шаблон:Nowrap. 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 with p being prime has characteristic p.
- 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
- The prime field of the field F is the unique smallest subfield of F.
- Extension field
- If F is a subfield of E then E is an extension field of F. We then also say that E/F is a field extension.
- Degree of an extension
- Given an extension E/F, the field E can be considered as a vector space over the field F, and the dimension of this vector space is the degree of the extension, denoted by [E : F].
- Finite extension
- A finite extension is a field extension whose degree is finite.
- Algebraic extension
- If an element α of an extension field E over F is the root of a non-zero polynomial in F[x], then α is algebraic over F. If every element of E is algebraic over F, then E/F is an algebraic extension.
- Generating set
- Given a field extension E/F and a subset S of E, we write F(S) for the smallest subfield of E that contains both F and S. It consists of all the elements of E that can be obtained by repeatedly using the operations +, −, *, / on the elements of F and S. If Шаблон:Nowrap, we say that E is generated by S over F.
- Primitive element
- An 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. Such an extension is called a simple extension.
- 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.
- Perfect field
- A field such that every finite extension is separable. All fields of characteristic zero, and all finite fields, are perfect.
- Imperfect degree
- Let F be a field of characteristic Шаблон:Nowrap; then Fp is a subfield. The degree Шаблон:Nowrap is called the imperfect degree of F. The field F is perfect if and only if its imperfect degree is 1. For example, if F is a function field of n variables over a finite field of characteristic Шаблон:Nowrap, then its imperfect degree is pn.Шаблон:Sfn
- Algebraically closed field
- A field F is algebraically closed if every polynomial in F[x] has a root in F; equivalently: every polynomial in F[x] is a product of linear factors.
- Algebraic closure
- An algebraic closure of a field F is an algebraic extension of F which is algebraically closed. Every field has an algebraic closure, and it is unique up to an isomorphism that fixes F.
- Transcendental
- Those elements of an extension field of F that are not algebraic over F are transcendental over F.
- Algebraically independent elements
- Elements of an extension field of F are algebraically independent over F if they don't satisfy any non-zero polynomial equation with coefficients in F.
- Transcendence degree
- The number of algebraically 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 ring homomorphism, i.e., a function
- f : E → F
- such that, for all x, y in E,
- f(x + y) = f(x) + f(y)
- f(xy) = f(x) f(y)
- f(1) = 1.
- For fields E and F, these properties imply that Шаблон:Nowrap, Шаблон:Nowrap for x in E×, 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 : E → F.
- The two fields are then identical for all practical purposes; however, not necessarily in a unique way. See, for example, Complex conjugate.
Types of fields
- Finite field
- A field with finitely many elements, a.k.a. Galois field.
- Ordered field
- A field with a total order compatible with its operations.
- Rational numbers
- Real numbers
- Complex numbers
- Number field
- Finite 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.
- Quadratic field
- 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
- Global field
- A number field or a function field of one variable over a finite field.
- Local field
- A completion of some global field (w.r.t. a prime of the integer ring).
- Complete field
- A field complete w.r.t. to some valuation.
- Pseudo algebraically closed field
- A field in which every variety has a rational point.Шаблон:Sfn
- Henselian field
- A field satisfying Hensel lemma w.r.t. some valuation. A generalization of complete fields.
- Hilbertian field
- A field satisfying Hilbert's irreducibility theorem: formally, one for which the projective line is not thin in the sense of Serre.Шаблон:SfnШаблон:Sfn
- Kroneckerian field
- A totally real algebraic number field or a totally imaginary quadratic extension of a totally real field.Шаблон:Sfn
- CM-field or J-field
- An algebraic number field which is a totally imaginary quadratic extension of a totally real field.Шаблон:Sfn
- Linked field
- A field over which no biquaternion algebra is a division algebra.Шаблон:Sfn
- Frobenius field
- A pseudo algebraically closed field whose absolute Galois group has the embedding property.Шаблон:Sfn
Field extensions
Let E/F be a field extension.
- Algebraic extension
- An extension in which every element of E is algebraic over F.
- Simple extension
- An extension which is generated by a single element, called a primitive element, or generating element.Шаблон:Sfn The primitive element theorem classifies such extensions.Шаблон:Sfn
- Normal extension
- An extension that splits a family of polynomials: every root of the minimal polynomial of an element of E over F is also in E.
- Separable extension
- An algebraic extension in which the minimal polynomial of every element of E over F is a separable polynomial, that is, has distinct roots.Шаблон:Sfn
- Galois extension
- A normal, separable field extension.
- Primary extension
- An extension E/F such that the algebraic closure of F in E is purely inseparable over F; equivalently, E is linearly disjoint from the separable closure of F.Шаблон:Sfn
- Purely transcendental extension
- An extension E/F in which every element of E not in F is transcendental over F.Шаблон:SfnШаблон:Sfn
- Regular extension
- An extension E/F such that E is separable over F and F is algebraically closed in E.Шаблон:Sfn
- Simple radical extension
- A simple extension E/F generated by a single element α satisfying Шаблон:Nowrap for an element b of F. In characteristic p, we also take an extension by a root of an Artin–Schreier polynomial to be a simple radical extension.Шаблон:Sfn
- Radical extension
- A tower Шаблон:Nowrap where each extension Шаблон:Nowrap is a simple radical extension.Шаблон:Sfn
- Self-regular extension
- An extension E/F such that Шаблон:Nowrap is an integral domain.Шаблон:Sfn
- Totally transcendental extension
- An extension E/F such that F is algebraically closed in F.Шаблон:Sfn
- Distinguished class
- A class C of field extensions with the three propertiesШаблон:Sfn
- If E is a C-extension of F and F is a C-extension of K then E is a C-extension of K.
- If E and F are C-extensions of K in a common overfield M, then the compositum EF is a C-extension of K.
- If E is a C-extension of F and Шаблон:Nowrap then E is a C-extension of K.
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 nth 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.
Citations
References
- Шаблон:Cite book
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- Шаблон:Lang Algebra
- Шаблон:Cite book
- Шаблон:Cite book
- Шаблон:Cite book
- Шаблон:Cite book
- Шаблон:Cite book
