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In mathematics, the **ring of integers** of an algebraic number field is the ring of all algebraic integers contained in .^{[1]} An algebraic integer is a root of a monic polynomial with integer coefficients: .^{[2]} This ring is often denoted by or . Since any integer belongs to and is an integral element of , the ring is always a subring of .

The ring of integers is the simplest possible ring of integers.^{[a]} Namely, where is the field of rational numbers.^{[3]} And indeed, in algebraic number theory the elements of are often called the "rational integers" because of this.

The next simplest example is the ring of Gaussian integers , consisting of complex numbers whose real and imaginary parts are integers. It is the ring of integers in the number field of Gaussian rationals, consisting of complex numbers whose real and imaginary parts are rational numbers. Like the rational integers, is a Euclidean domain.

The ring of integers of an algebraic number field is the unique maximal order in the field. It is always a Dedekind domain.^{[4]}

The ring of integers O_{K} is a finitely-generated **Z**-module. Indeed, it is a free **Z**-module, and thus has an **integral basis**, that is a basis *b*_{1}, ..., *b*_{n} ∈ O_{K} of the **Q**-vector space K such that each element x in O_{K} can be uniquely represented as

with *a*_{i} ∈ **Z**.^{[5]} The rank n of O_{K} as a free **Z**-module is equal to the degree of K over **Q**.

A useful tool for computing the integral closure of the ring of integers in an algebraic field *K*/**Q** is the discriminant. If *K* is of degree *n* over **Q**, and form a basis of *K* over **Q**, set . Then, is a submodule of the **Z**-module spanned by .^{[6]} ^{pg. 33} In fact, if *d* is square-free, then forms an integral basis for .^{[6]} ^{pg. 35}

If p is a prime, *ζ* is a pth root of unity and *K* = **Q**(*ζ* ) is the corresponding cyclotomic field, then an integral basis of O_{K} = **Z**[*ζ*] is given by (1, *ζ*, *ζ*^{ 2}, ..., *ζ*^{ p−2}).^{[7]}

If is a square-free integer and is the corresponding quadratic field, then is a ring of quadratic integers and its integral basis is given by (1, (1 + √*d*) /2) if *d* ≡ 1 (mod 4) and by (1, √*d*) if *d* ≡ 2, 3 (mod 4).^{[8]} This can be found by computing the minimal polynomial of an arbitrary element where .

In a ring of integers, every element has a factorization into irreducible elements, but the ring need not have the property of unique factorization: for example, in the ring of integers **Z**[√−5], the element 6 has two essentially different factorizations into irreducibles:^{[4]}^{[9]}

A ring of integers is always a Dedekind domain, and so has unique factorization of ideals into prime ideals.^{[10]}

The units of a ring of integers *O*_{K} 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*.^{[11]}

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.^{[12]} 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.^{[3]}

For example, the p-adic integers **Z**_{p} are the ring of integers of the p-adic numbers **Q**_{p} .

- Minimal polynomial (field theory)
- Integral closure – gives a technique for computing integral closures

**^***The ring of integers*, without specifying the field, refers to the ring of "ordinary" integers, the prototypical object for all those rings. It is a consequence of the ambiguity of the word "integer" in abstract algebra.

**^**Alaca & Williams 2003, p. 110, Defs. 6.1.2-3.**^**Alaca & Williams 2003, p. 74, Defs. 4.1.1-2.- ^
^{a}^{b}Cassels 1986, p. 192. - ^
^{a}^{b}Samuel 1972, p. 49. **^**Cassels (1986) p. 193- ^
^{a}^{b}Baker. "Algebraic Number Theory" (PDF). pp. 33–35. **^**Samuel 1972, p. 43.**^**Samuel 1972, p. 35.**^**Artin, Michael (2011).*Algebra*. Prentice Hall. p. 360. ISBN 978-0-13-241377-0.**^**Samuel 1972, p. 50.**^**Samuel 1972, pp. 59–62.**^**Cassels 1986, p. 41.

- Alaca, Saban; Williams, Kenneth S. (2003).
*Introductory Algebraic Number Theory*. Cambridge University Press. ISBN 9780511791260. - Cassels, J.W.S. (1986).
*Local fields*. London Mathematical Society Student Texts. Vol. 3. Cambridge: Cambridge University Press. ISBN 0-521-31525-5. Zbl 0595.12006. - Neukirch, Jürgen (1999).
*Algebraische Zahlentheorie*.*Grundlehren der mathematischen Wissenschaften*. Vol. 322. Berlin: Springer-Verlag. ISBN 978-3-540-65399-8. MR 1697859. Zbl 0956.11021. - Samuel, Pierre (1972).
*Algebraic number theory*. Hermann/Kershaw.