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Integral domain
From Wikipedia, the free encyclopedia

In mathematics, and specifically in abstract algebra, an integral domain is a nonzero commutative
ring in which the product of any two nonzero elements is nonzero.[1][2] Integral domains are
generalizations of the ring of integers and provide a natural setting for studying divisibility. In an
integral domain the cancellation property holds for multiplication by a nonzero element a, that is, if a
≠ 0, an equality ab = ac implies b = c.
"Integral domain" is defined almost universally as above, but there is some variation. This article
follows the convention that rings have a 1, but some authors who do not follow this also do not
require integral domains to have a 1.[3][4] Noncommutative integral domains are sometimes
admitted.[5] This article, however, follows the much more usual convention of reserving the term
"integral domain" for the commutative case and using "domain" for the general case including
noncommutative rings.
Some sources, notably Lang, use the term entire ring for integral domain.[6]
Some specific kinds of integral domains are given with the following chain of class inclusions:
Commutative rings ⊃ integral domains ⊃ integrally closed domains ⊃ unique
factorization domains ⊃ principal ideal domains ⊃ Euclidean domains ⊃ fields

Algebraic structures
Group-like[show]
Ring-like[show]
Lattice-like[show]
Module-like[show]
Algebra-like[show]

Contents
[hide]












1 Definitions
2 Examples
3 Non-examples
4 Divisibility, prime elements, and irreducible elements
5 Properties
6 Field of fractions
7 Algebraic geometry
8 Characteristic and homomorphisms
9 See also
10 Notes
11 References



V



T



E

Definitions[edit]
There are a number of equivalent definitions of integral domain:









An integral domain is a nonzero commutative ring in which the product of any two nonzero
elements is nonzero.
An integral domain is a nonzero commutative ring with no nonzero zero divisors.
An integral domain is a commutative ring in which the zero ideal {0} is a prime ideal.
An integral domain is a commutative ring for which every non-zero element is cancellable
under multiplication.
An integral domain is a ring for which the set of nonzero elements is a
commutative monoid under multiplication (because the monoid is closed under
multiplication).
An integral domain is a ring that is (isomorphic to) a subring of a field. (This implies it is
a nonzero commutative ring.)
An integral domain is a nonzero commutative ring in which for every nonzero element r, the
function that maps each element x of the ring to the product xr is injective. Elements r with
this property are called regular, so it is equivalent to require that every nonzero element of
the ring be regular.

Examples[edit]



The archetypical example is the ring Z of all integers.
Every field is an integral domain. Conversely, every Artinian integral domain is a field. In
particular, all finite integral domains are finite fields (more generally, by Wedderburn's little
theorem, finite domains are finite fields). The ring of integers Z provides an example of a
non-Artinian infinite integral domain that is not a field, possessing infinite descending
sequences of ideals such as:











Rings of polynomials are integral domains if the coefficients come from an integral
domain. For instance, the ring Z[X] of all polynomials in one variable with integer
coefficients is an integral domain; so is the ring R[X,Y] of all polynomials in two variables
with real coefficients.
For each integer n > 1, the set of all real numbers of the
form a + b√n with a and b integers is a subring of R and hence an integral domain.
For each integer n > 0 the set of all complex numbers of the
form a + bi√n with a and b integers is a subring of C and hence an integral domain. In
the case n = 1 this integral domain is called the Gaussian integers.
The ring of p-adic integers is an integral domain.
If U is a connected open subset of the complex plane C, then the ring H(U) consisting of
all holomorphic functions f : U → C is an integral domain. The same is true for rings
ofanalytic functions on connected open subsets of analytic manifolds.
A regular local ring is an integral domain. In fact, a regular local ring is a UFD.[7][8]

Non-examples[edit]
The following rings are not integral domains.


The ring of n × n matrices over any nonzero ring when n ≥ 2.






The ring of continuous functions on the unit interval.
The quotient ring Z/mZ when m is a composite number.
The product ring Z × Z.
The zero ring in which 0=1.



The tensor product

(since, for

example,


).

The quotient ring

for any field , since

is not a prime ideal.

Divisibility, prime elements, and irreducible elements[edit]
See also: Divisibility (ring theory)
In this section, R is an integral domain.
Given elements a and b of R, we say that a divides b, or that a is a divisor of b, or that b is
a multiple of a, if there exists an element x in R such that ax = b.
The elements that divide 1 are called the units of R; these are precisely the invertible
elements in R. Units divide all other elements.
If a divides b and b divides a, then we say a and b are associated
elements or associates.[9] Equivalently, a and b are associates if a=ub for some unit u.
If q is a nonzero non-unit, we say that q is an irreducible element if q cannot be written as
a product of two non-units.
If p is a nonzero non-unit, we say that p is a prime element if, whenever p divides a
product ab, then p divides a or p divides b. Equivalently, an element p is prime if and only if
the principal ideal (p) is a nonzero prime ideal. The notion of prime element generalizes the
ordinary definition of prime number in the ring Z, except that it allows for negative prime
elements.
Every prime element is irreducible. The converse is not true in general: for example, in
the quadratic integer ring
the element 3 is irreducible (if it factored nontrivially,
the factors would each have to have norm 3, but there are no norm 3 elements
since

has no integer solutions), but not prime (since 3

divides
without dividing either factor). In a unique
factorization domain (or more generally, a GCD domain), an irreducible element is a prime
element.
While unique factorization does not hold in
of ideals. See Lasker–Noether theorem.

, there is unique factorization

Properties[edit]




A commutative ring R is an integral domain if and only if the ideal (0) of R is a prime
ideal.
If R is a commutative ring and P is an ideal in R, then the quotient ring R/P is an integral
domain if and only if P is a prime ideal.
Let R be an integral domain. Then there is an integral domain S such
that R ⊂ S and S has an element which is transcendental over R.







The cancellation property holds in any integral domain: for any a, b, and c in an integral
domain, if a ≠ 0 and ab = ac then b = c. Another way to state this is that the
function x↦ ax is injective for any nonzero a in the domain.
The cancellation property holds for ideals in any integral domain: if xI = xJ, then
either x is zero or I = J.
An integral domain is equal to the intersection of its localizations at maximal ideals.
An inductive limit of integral domains is an integral domain.

Field of fractions[edit]
Main article: Field of fractions
The field of fractions K of an integral domain R is the set of
fractions a/b with a and b in R and b ≠ 0 modulo an appropriate equivalence relation,
equipped with the usual addition and multiplication operations. It is "the smallest field
containing R" in the sense that there is an injective ring homomorphism R → K such that
any injective ring homomorphism from R to a field factors through K. The field of fractions of
the ring of integers Z is the field of rational numbers Q. The field of fractions of a field
is isomorphic to the field itself.

Algebraic geometry[edit]
Integral domains are characterized by the condition that they are reduced (that is x2 = 0
implies x = 0) and irreducible (that is there is only one minimal prime ideal). The former
condition ensures that the nilradical of the ring is zero, so that the intersection of all the
ring's minimal primes is zero. The latter condition is that the ring have only one minimal
prime. It follows that the unique minimal prime ideal of a reduced and irreducible ring is the
zero ideal, so such rings are integral domains. The converse is clear: an integral domain has
no nonzero nilpotent elements, and the zero ideal is the unique minimal prime ideal.
This translates, in algebraic geometry, into the fact that the coordinate ring of an affine
algebraic set is an integral domain if and only if the algebraic set is an algebraic variety.
More generally, a commutative ring is an integral domain if and only if its spectrum is
an integral affine scheme.

Characteristic and homomorphisms[edit]
The characteristic of an integral domain is either 0 or a prime number.
If R is an integral domain of prime characteristic p, then the Frobenius endomorphism f(x)
= x p is injective.

See also[edit]




Integral domains – wikibook link
Dedekind–Hasse norm – the extra structure needed for an integral domain to be
principal
Zero-product property

Notes[edit]
1. Jump up^ Bourbaki, p. 116.

2. Jump up^ Dummit and Foote, p. 228.
3. Jump up^ B.L. van der Waerden, Algebra Erster Teil, p. 36, Springer-Verlag, Berlin,
Heidelberg 1966.
4. Jump up^ I.N. Herstein, Topics in Algebra, p. 88-90, Blaisdell Publishing Company,
London 1964.
5. Jump up^ J.C. McConnel and J.C. Robson "Noncommutative Noetherian Rings"
(Graduate studies in Mathematics Vol. 30, AMS)
6. Jump up^ Pages 91–92 of Lang, Serge (1993), Algebra (Third ed.), Reading,
Mass.: Addison-Wesley Pub. Co., ISBN 978-0-201-55540-0, Zbl 0848.13001
7. Jump up^ Auslander, Maurice; Buchsbaum, D. A. (1959). "Unique factorization in
regular local rings". Proc. Natl. Acad. Sci. USA 45 (5): 733–
734. doi:10.1073/pnas.45.5.733. PMC 222624.PMID 16590434.
8. Jump up^ Masayoshi Nagata (1958). "A general theory of algebraic geometry over
Dedekind domains. II". Amer. J. Math. (The Johns Hopkins University Press) 80 (2):
382–420.doi:10.2307/2372791. JSTOR 2372791.
9. Jump up^ Durbin, John R. (1993). Modern Algebra: An Introduction (3rd ed.). John
Wiley and Sons. p. 224. ISBN 0-471-51001-7. Elements a and b of [an integral domain]
are called associates if a | band b | a.

References[edit]












Adamson, Iain T. (1972). Elementary rings and modules. University Mathematical Texts.
Oliver and Boyd. ISBN 0-05-002192-3.
Bourbaki, Nicolas (1998). Algebra, Chapters 1–3. Berlin, New York: SpringerVerlag. ISBN 978-3-540-64243-5.
Mac Lane, Saunders; Birkhoff, Garrett (1967). Algebra. New York: The Macmillan
Co. ISBN 1-56881-068-7. MR 0214415.
Dummit, David S.; Foote, Richard M. (2004). Abstract Algebra (3rd ed.). New
York: Wiley. ISBN 978-0-471-43334-7.
Hungerford, Thomas W. (1974). Algebra. New York: Holt, Rinehart and Winston,
Inc. ISBN 0-03-030558-6.
Lang, Serge (2002). Algebra. Graduate Texts in Mathematics 211. Berlin, New
York: Springer-Verlag. ISBN 978-0-387-95385-4. MR 1878556.
Sharpe, David (1987). Rings and factorization. Cambridge University Press. ISBN 0521-33718-6.
Rowen, Louis Halle (1994). Algebra: groups, rings, and fields. A K Peters. ISBN 156881-028-8.
Lanski, Charles (2005). Concepts in abstract algebra. AMS Bookstore. ISBN 0-53442323-X.
Milies, César Polcino; Sehgal, Sudarshan K. (2002). An introduction to group rings.
Springer. ISBN 1-4020-0238-6.
B.L. van der Waerden, Algebra, Springer-Verlag, Berlin Heidelberg, 1966.

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