Algebraic integer

Algebraic integer

"This article deals with the ring of complex numbers integral over" Z. "For the general notion of algebraic integer, see Integrality".

In number theory, an algebraic integer is a complex number which is a root of some monic polynomial (leading coefficient 1) with integer coefficients. The set of all algebraic integers is closed under addition and multiplication so it forms a subring of complex numbers denoted by A. The ring A is the integral closure of regular integers in complex numbers.

The ring of integers of a number field "K", denoted by "OK" , is the intersection of "K" and A: it may also be characterised as the maximal order of the field "K".Each algebraic integer belongs to the ring of integers of some number field. A number "x" is an algebraic integer if and only if the ring Z ["x"] is finitely generated as an abelian group, which is to say, a Z-module.

Examples

* The only algebraic integers in rational numbers are the ordinary integers. In other words, the intersection of Q and A is exactly Z. The rational number "a"/"b" is not an algebraic integer unless "b" divides "a". Note that the leading coefficient of the polynomial "bx" − "a" is the integer "b". As another special case, the square root √"n" of a non-negative integer "n" is an algebraic integer, and so is irrational unless "n" is a perfect square.
*If "d" is a square free integer then the extension "K" = Q(√"d") is a quadratic field extension of rational numbers. The ring of algebraic integers "OK" contains √"d" since this is a root of the monic polynomial "x"² − "d". Moreover, if "d" ≡ 1 (mod 4) the element (1 + √"d")/2 is also an algebraic integer. It satisfies the polynomial "x"² − "x" + (1 − "d")/4 where the constant term (1 − "d")/4 is an integer. The full ring of integers is generated by √"d" or (1 + √"d")/2 respectively.
* If zeta_n is a primitive "n"-th root of unity, then the ring of integers of the cyclotomic field mathbf{Q}(zeta) is precisely mathbf{Z} [zeta] .
* If "α" is an algebraic integer then eta=sqrt [n] {alpha} is another algebraic integer. A polynomial for "β" is obtained by substituting "x""n" in the polynomial for "α".

Non-example

* If "P"("x") is a primitive polynomial which has integer coefficients but is not monic, and "P" is irreducible over Q, then none of the roots of "P" are algebraic integers. (Here "primitive" is used in the sense that the highest common factor of the set of coefficients of "P" is 1; this is weaker than requiring the coefficients to be pairwise relatively prime.)

Facts

* The sum, difference and product of two algebraic integers is an algebraic integer. In general their quotient is not. The monic polynomial involved is generally of higher degree than those of the original algebraic integers, and can be found by taking resultants and factoring. For example, if "x"² − "x" − 1 = 0, "y"³ − "y" − 1 = 0 and "z" = "xy", then eliminating "x" and "y" from "z" − "xy" and the polynomials satisfied by "x" and "y" using the resultant gives "z"6 − 3"z"4 − 4"z"³ + "z"² + "z" − 1, which is irreducible, and is the monic polynomial satisfied by the product. (To see that the "xy" is a root of the x-resultant of "z" − "xy" and "x"² − "x" − 1, one might use the fact that the resultant is contained in the ideal generated by its two input polynomials.)

* Any number constructible out of the integers with roots, addition, and multiplication is therefore an algebraic integer; but not all algebraic integers are so constructible: most roots of irreducible quintics are not.

* Every root of a monic polynomial whose coefficients are algebraic integers is itself an algebraic integer. In other words, the algebraic integers form a ring which is integrally closed in any of its extension.

* The ring of algebraic integers A is a Bézout domain.

References

* Daniel A. Marcus, "Number Fields", third edition, Springer-Verlag, 1977

ee also

*Integrality
*Gaussian integer
*Eisenstein integer
*Root of unity
*Dirichlet's unit theorem
*Fundamental units


Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • algebraic integer — noun A number (real or complex) which is a solution to an equation of the form for some set of integers through . See Also: quadratic integer …   Wiktionary

  • Algebraic number field — In mathematics, an algebraic number field (or simply number field) F is a finite (and hence algebraic) field extension of the field of rational numbers Q. Thus F is a field that contains Q and has finite dimension when considered as a vector… …   Wikipedia

  • Algebraic number — In mathematics, an algebraic number is a complex number that is a root of a non zero polynomial in one variable with rational (or equivalently, integer) coefficients. Complex numbers such as pi that are not algebraic are said to be transcendental …   Wikipedia

  • Integer — This article is about the mathematical concept. For integers in computer science, see Integer (computer science). Symbol often used to denote the set of integers The integers (from the Latin integer, literally untouched , hence whole : the word… …   Wikipedia

  • Algebraic number theory — In mathematics, algebraic number theory is a major branch of number theory which studies the algebraic structures related to algebraic integers. This is generally accomplished by considering a ring of algebraic integers O in an algebraic number… …   Wikipedia

  • Algebraic modeling language — Algebraic Modeling Languages (AML) are high level programming languages for describing and solving high complexity problems for large scale mathematical computation (i.e. large scale optimization type problems). One particular advantage of AMLs… …   Wikipedia

  • Algebraic-group factorisation algorithm — Algebraic group factorisation algorithms are algorithms for factoring an integer N by working in an algebraic group defined modulo N whose group structure is the direct sum of the reduced groups obtained by performing the equations defining the… …   Wikipedia

  • Algebraic curve — In algebraic geometry, an algebraic curve is an algebraic variety of dimension one. The theory of these curves in general was quite fully developed in the nineteenth century, after many particular examples had been considered, starting with… …   Wikipedia

  • Algebraic data type — In computer programming, particularly functional programming and type theory, an algebraic data type (sometimes also called a variant type[1]) is a datatype each of whose values is data from other datatypes wrapped in one of the constructors of… …   Wikipedia

  • Algebraic structure — In algebra, a branch of pure mathematics, an algebraic structure consists of one or more sets closed under one or more operations, satisfying some axioms. Abstract algebra is primarily the study of algebraic structures and their properties. The… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”