Algebraic number

In mathematics, an algebraic number is a complex number that is an algebraic element over the rational numbers. In other words, an algebraic number is a root of a non-zero polynomial with rational (or equivalently, integer) coefficients.

Examples

• All rational numbers are algebraic.
• The irrational numbers $\textstyle {\sqrt {2}}$ (the square root of 2) and $\textstyle {\frac {1}{2}}{\sqrt[{3}]{3}}$ (half the cube root of 3) are algebraic because they are the roots of x² − 2 = 0 and 8x³ − 3 = 0, respectively. The imaginary unit i is algebraic, since it satisfies x² + 1 = 0.
• Real numbers π and e are not algebraic(see the Lindemann–Weierstrass theorem).

Properties

• The set of algebraic numbers is countable.
• Given an algebraic number, there is a unique monic polynomial of least degree that has the number as a root. This polynomial is called its minimal polynomial. If its minimal polynomial has degree n, then the algebraic number is said to be of degree n. An algebraic number of degree 1 is a rational number.
• All algebraic numbers are computable and therefore definable.

The field of algebraic numbers

The sum, difference, product and quotient of two algebraic numbers is again algebraic, and the algebraic numbers therefore form a field, sometimes denoted by $\mathbb {A}$ (which may also denote the adele ring) or ${\overline {\mathbb {Q} }}$. It can be shown that every root of a polynomial equation whose coefficients are algebraic numbers is again algebraic. This can be rephrased by saying that the field of algebraic numbers is algebraically closed. In fact, it is the smallest algebraically closed field containing the rationals, and is therefore called the algebraic closure of the rationals.

All the above statements are most easily proved in the general context of algebraic elements of a field extension.

Numbers defined by radicals

All numbers which can be obtained from the integers using a finite number of additions, subtractions, multiplications, divisions, and taking n^th roots (where n is a positive integer) are algebraic. The converse, however, is not true: there are algebraic numbers which cannot be obtained in this manner. All of these numbers are solutions to polynomials of degree ≥ 5. This is a result of Galois theory (see Quintic equations and the Abel–Ruffini theorem). An example of such a number is the unique real root of x⁵ − x − 1 = 0.

==Algebraic integers==

Main article: algebraic integer

An algebraic integer is a number which is a root of a polynomial with integer coefficients (that is, an algebraic number) with leading coefficient 1 (a monic polynomial). Examples of algebraic integers are 3√2 + 5 and 6i - 2.

The sum, difference and product of algebraic integers are again algebraic integers, which means that the algebraic integers form a ring. The name algebraic integer comes from the fact that the only rational numbers which are algebraic integers are the integers, and because the algebraic integers in any number field are in many ways analogous to the integers. If K is a number field, its ring of integers is the subring of algebraic integers in K, and is frequently denoted as O_K. These are the prototypical examples of Dedekind domains.

Special classes of algebraic number

• Gaussian integer
• Eisenstein integer
• Quadratic irrational
• Fundamental unit
• Root of unity
• Gaussian period
• Pisot-Vijayaraghavan number
• Salem number