nLab algebraic number

Definition

An algebraic number is a root of a non-zero polynomial with integer coefficients (or, equivalently, with rational coefficients). Equivalently, an element $\alpha$ of a field extension $K$ of the rational numbers $\mathbb{Q}$ is algebraic if the subfield $\mathbb{Q}(\alpha)$ is a finite degree extension, i.e., is finite-dimensional as a vector space over $\mathbb{Q}$.

Since the rational numbers are a subfield of the complex numbers, and since the complex numbers are an algebraically closed field, algebraic numbers are naturally regarded as a sub-field of complex numbers

$$\mathbb{Q} \hookrightarrow \mathbb{C} \,.$$

$,$

Visualisation of the (countable) field of algebraic numbers in the complex plane. Colours indicate degree of the polynomial the number is a root of (red = linear, i.e. the rationals, green = quadratic, blue = cubic, yellow = quartic…). Points becomes smaller as the integer polynomial coefficients become larger. View shows integers 0,1 and 2 at bottom right, $+i$ near top.

due to Stephen J. Brooks here

But the collection of all algebraic numbers forms itself already an algebraically closed field, typically denoted $\overline{\mathbb{Q}}$, as this is the algebraic closure of the field $\mathbb{Q}$ of rational numbers. This also follows easily from the equivalent definition of algebraic numbers in terms of finite degree extensions. The absolute Galois group $Gal(\overline{\mathbb{Q}}, \mathbb{Q})$ is peculiar, see there.

Given a field $k$, an algebraic number field $K$ over $k$ is a finite-degree extension of $k$. By default, the term “algebraic number field” means an algebraic number field over the rational numbers. If $\alpha$ is an algebraic number over $\mathbb{Q}$ then $\mathbb{Q}[\alpha]$ is a number field, however the field of all algebraic numbers is not a number field.

An algebraic integer is a root of a monic polynomial with integer coefficients. Equivalently, an element $\alpha$ of a field extension $K$ of $\mathbb{Q}$ is an algebraic integer if the ring $\mathbb{Z}[\alpha]$ is of finite rank as a $\mathbb{Z}$-module. It follows easily from this characterization that the collection of all algebraic integers forms a commutative ring.

Related concepts

• algebraic number theory

  • algebraic number, algebraic integer

  • number field

• transcendental number

References

See also

• Wikipedia, Algebraic numbers