Ring of integers

In mathematics, the ring of integers of an algebraic number field Template:Mvar is the ring of all integral elements contained in Template:Mvar. An integral element is a root of a monic polynomial with rational integer coefficients, xn + cn−1xn−1 + … + c0. This ring is often denoted by OK or ${\mathcal {O}}_{K}$ . Since any rational integer number belongs to Template:Mvar and is its integral element, the ring Z is always a subring of OK.

The ring Z is the simplest possible ring of integers. Namely, Z = OQ where Q is the field of rational numbers. And indeed, in algebraic number theory the elements of Z are often called the "rational integers" because of this.

The ring of integers of a algebraic number field is the unique maximal order in the field.

Properties

The ring of integers OK is a finitely-generated Z-module. Indeed it is a free Z-module, and thus has an integral basis, that is a basis b1, … ,bn ∈ OK of the Q-vector space Template:Mvar such that each element Template:Mvar in OK can be uniquely represented as

$x=\sum _{i=1}^{n}a_{i}b_{i},$ with aiZ. The rank Template:Mvar of OK as a free Z-module is equal to the degree of Template:Mvar over Q.

The rings of integers in number ﬁelds are Dedekind domains.

Examples

If Template:Mvar is a prime, ζ is a Template:Mvarth root of unity and K = Q(ζ) is the corresponding cyclotomic field, then an integral basis of OK = Z[ζ] is given by (1, ζ, ζ2, … , ζp−2).

If Template:Mvar is a square-free integer and K = Q(Template:Sqrt) is the corresponding quadratic field, then OK is a ring of quadratic integers and its integral basis is given by (1, (1 + Template:Sqrt)/2) if d ≡ 1 (mod 4) and by (1, Template:Sqrt) if d ≡ 2, 3 (mod 4).

Multiplicative structure

In a ring of integers, every element has a factorisation into irreducible elements, but the ring need not have the property of unique factorisation: for example, in the ring of integers ℤ[√-5] the element 6 has two essentially different factorisations into irreducibles:

$6=2\cdot 3=(1+{\sqrt {-5}})(1-{\sqrt {-5}})\ .$ A ring of integers is always a Dedekind domain, and so has unique factorisation of ideals into prime ideals.

The units of a ring of integers OK is a finitely generated abelian group by Dirichlet's unit theorem. The torsion subgroup consists of the roots of unity of K. A set of torsion-free generators is called a set of fundamental units.

Generalization

One defines the ring of integers of a non-archimedean local field F as the set of all elements of F with absolute value ≤1; this is a ring because of the strong triangle inequality. If F is the completion of an algebraic number field, its ring of integers is the completion of the latter's ring of integers. The ring of integers of an algebraic number field may be characterised as the elements which are integers in every non-archimedean completion.

For example, the Template:Mvar-adic integers Zp are the ring of integers of the [[p-adic number|Template:Mvar-adic numbers]] Qp.