# Ideal (ring theory)

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Ideal (ring theory)

In ring theory, a branch of abstract algebra, an ideal is a special subset of a ring. The ideal concept allows the generalization in an appropriate way of some important properties of integers like "even number" or "multiple of 3".

For instance, in rings one studies prime ideals instead of prime numbers, one defines coprime ideals as a generalization of coprime numbers, and one can prove a generalized Chinese remainder theorem about ideals. In a certain class of rings important in number theory, the Dedekind domains, one can even recover a version of the fundamental theorem of arithmetic: in these rings, every nonzero ideal can be uniquely written as a product of prime ideals.

An ideal can be used to construct a quotient ring in a similar way as a normal subgroup in group theory can be used to construct a quotient group. The concept of an order ideal in order theory is derived from the notion of ideal in ring theory.

A fractional ideal is a generalization of an ideal, and the usual ideals are sometimes called integral ideals for clarity.

## History

Ideals were first proposed by Richard Dedekind in 1876 in the third edition of his book Vorlesungen über Zahlentheorie (English: Lectures on Number Theory). They were a generalization of the concept of ideal numbers developed by Ernst Kummer.  Later the concept was expanded by David Hilbert and especially Emmy Noether.

## Definitions

For an arbitrary ring $(R,+,\cdot)$, let (R, + ) be the underlying additive group. A subset I is called a two-sided ideal (or simply an ideal) of R if I is an additive subgroup of R that "absorbs multiplication by elements of R".. Formally we mean that I is an ideal if it satisfies the following conditions:

1. (I, + ) is a subgroup of (R, + )
2. $\forall x \isin I, \forall r \isin R :\quad x \cdot r \isin I$
3. $\forall x \isin I, \forall r \isin R : \quad r \cdot x \isin I.$

Equivalently, an ideal of R is a sub-R-bimodule of R.

A subset I of R is called a right ideal of R  if it is an additive subgroup of R and absorbs multiplication on the right, that is:

1. (I, + ) is a subgroup of (R, + )
2. $\forall x \isin I, \forall r \isin R: \quad x \cdot r \isin I.$

Equivalently, a right ideal of R is a right R-submodule of R.

Similarly a subset I of R is called a left ideal of R if it is an additive subgroup of R absorbing multiplication on the left:

1. (I, + ) is a subgroup of (R, + )
2. $\forall x \isin I, \forall r \isin R: \quad r \cdot x \isin I.$

Equivalently, a left ideal of R is a left R-submodule of R.

In all cases, the first condition can be replaced by the following well-known criterion that ensures a nonempty subset of a group is a subgroup:

1.' I is non-empty and $\forall_{x,y \isin I}~ x - y \isin I$.

The left ideals in R are exactly the right ideals in the opposite ring Ro and vice versa. A two-sided ideal is a left ideal that is also a right ideal, and is often called an ideal except to emphasize that there might exist single-sided ideals. When R is a commutative ring, the definitions of left, right, and two-sided ideal coincide, and the term ideal is used alone.

Just as normal subgroups of groups are kernels of group homomorphisms, left/right/two-sided ideals have interpretations as kernels. For a nonempty subset A of R:

• A is an ideal of R if and only if it is a kernel of a ring homomorphism from R.
• A is a right ideal of R if and only if it is a kernel of a homomorphism from the right R module RR to another right R module.
• A is a left ideal of R if and only if it is a kernel of a homomorphism from the left R module RR to another left R module.

If p is in R, then pR is a right ideal and Rp is a left ideal of R. These are called, respectively, the principal right and left ideals generated by p. To remember which is which, note that right ideals are stable under right-multiplication (IR ⊆ I) and left ideals are stable under left-multiplication (RI ⊆ I).

The connection between cosets and ideals can be seen by switching the operation from "multiplication" to "addition".

We call I a proper ideal if it is a proper subset of R, that is, I does not equal R. The ideal R is called the unit ideal.

## Motivation

Intuitively, the definition can be motivated as follows: Suppose we have a subset of elements Z of a ring R and that we would like to obtain a ring with the same structure as R, except that the elements of Z should be zero (they are in some sense "negligible").

But if z1 = 0 and z2 = 0 in our new ring, then surely z1 + z2 should be zero too, and rz1 as well as z1r should be zero for any element r (zero or not).

The definition of an ideal is such that the ideal I generated (see below) by Z is exactly the set of elements that are forced to become zero if Z becomes zero, and the quotient ring R/I is the desired ring where Z is zero, and only elements that are forced by Z to be zero are zero. The requirement that R and R/I should have the same structure (except that I becomes zero) is formalized by the condition that the projection from R to R/I is a (surjective) ring homomorphism.

## Examples

• The even integers form an ideal in the ring Z of all integers; it is usually denoted by 2Z. This is because the sum of any even integers is even, and the product of any integer with an even integer is also even. Similarly, the set of all integers divisible by a fixed integer n is an ideal denoted nZ.
• The set of all polynomials with real coefficients which are divisible by the polynomial x2 + 1 is an ideal in the ring of all polynomials.
• The set of all n-by-n matrices whose last row is zero forms a right ideal in the ring of all n-by-n matrices. It is not a left ideal. The set of all n-by-n matrices whose last column is zero forms a left ideal but not a right ideal.
• The ring C(R) of all continuous functions f from R to R contains the ideal of all continuous functions f such that f(1) = 0. Another ideal in C(R) is given by those functions which vanish for large enough arguments, i.e. those continuous functions f for which there exists a number L > 0 such that f(x) = 0 whenever |x| > L.
• {0} and R are ideals in every ring R. If R is a division ring or a field, then these are its only ideals.
• Compact operators form an ideal in the ring of bounded operators.

## Ideal generated by a set

Let R be a (possibly not unital) ring. Any intersection of any nonempty family of left ideals of R is again a left ideal of R. If X is any subset of R, then the intersection of all left ideals of R containing X is a left ideal I of R containing X, and is clearly the smallest left ideal to do so. This ideal I is said to be the left ideal generated by X. Similar definitions can be created by using right ideals or two-sided ideals in place of left ideals.

If R is commutative, the left, right, and two-sided ideals generated by a subset X of R are the same, since the left, right, and two-sided ideals of R are the same. We then speak of the ideal of R generated by X, without further specification. However, if R is not commutative they may not be the same.

If R has unity, then the left, right, or two-sided ideal of R generated by a subset X of R can be expressed internally as we will now describe. The following set is a left ideal: $\{r_1x_1+\dots+r_nx_n \mid n\in\mathbb{N}, r_i\in R, x_i\in X\}.\,$

Each element described would have to be in every left ideal containing X, so this left ideal is in fact the left ideal generated by X. The right ideal and ideal generated by X can also be expressed in the same way: $\{x_1r_1+\dots+x_nr_n \mid n\in\mathbb{N}, r_i\in R, x_i\in X\}\,$ $\{r_1x_1s_1+\dots+r_nx_ns_n \mid n\in\mathbb{N}, r_i\in R,s_i\in R, x_i\in X\}.\,$

The former is the right ideal generated by X, and the latter is the ideal generated by X.

By convention, 0 is viewed as the sum of zero such terms, agreeing with the fact that the ideal of R generated by ∅ is {0} by the previous definition.

If a left ideal I of R has a finite subset F such that I is the left ideal generated by F, then the left ideal I is said to be finitely generated. Similar terms are also applied to right ideals and two-sided ideals generated by finite subsets.

In the special case where the set X is just a singleton {a} for some a in R, then the above definitions turn into the following: $Ra=\{ra \mid r\in R\}\,$ $aR=\{ar \mid r\in R\}\,$ $RaR=\{r_1as_1+\dots+r_nas_n \mid n\in\mathbb{N}, r_i\in R,s_i\in R, x_i\in X\}.\,$

These ideals are known as the left/right/two-sided principal ideals generated by a. It is also very common to denote the two-sided ideal generated by a as (a).

If R does not have a unit, then the internal descriptions above must be modified slightly. In addition to the finite sums of products of things in X with things in R, we must allow the addition of n-fold sums of the form x+x+...+x, and n-fold sums of the form (−x)+(−x)+...+(−x) for every x in X and every n in the natural numbers. When R has a unit, this extra requirement becomes superfluous.

### Example

• In the ring Z of integers, every ideal can be generated by a single number (so Z is a principal ideal domain), and the only two generators of pR are p and −p. The concepts of "ideal" and "number" are therefore almost identical in Z. If aR = bR in an arbitrary domain, then au = b for some unit u. Conversely, for any unit u, aR = auu-1R = auR. So, in a commutative principal ideal domain, the generators of the ideal aR are just the elements au where u is an arbitrary unit. This explains the case of Z since 1 and −1 are the only units of Z.

## Types of ideals

To simplify the description all rings are assumed to be commutative. The non-commutative case is discussed in detail in the respective articles

Ideals are important because they appear as kernels of ring homomorphisms and allow one to define factor rings. Different types of ideals are studied because they can be used to construct different types of factor rings.

• Maximal ideal: A proper ideal I is called a maximal ideal if there exists no other proper ideal J with I a subset of J. The factor ring of a maximal ideal is a simple ring in general and is a field for commutative rings.
• Prime ideal: A proper ideal I is called a prime ideal if for any a and b in R, if ab is in I, then at least one of a and b is in I. The factor ring of a prime ideal is a prime ring in general and is an integral domain for commutative rings.
• Radical ideal or semiprime ideal: A proper ideal I is called radical or semiprime if for any a in R, if an is in I for some n, then a is in I. The factor ring of a radical ideal is a semiprime ring for general rings, and is a reduced ring for commutative rings.
• Primary ideal: An ideal I is called a primary ideal if for all a and b in R, if ab is in I, then at least one of a and bn is in I for some natural number n. Every prime ideal is primary, but not conversely. A semiprime primary ideal is prime.
• Principal ideal: An ideal generated by one element.
• Primitive ideal: A left primitive ideal is the annihilator of a simple left module. A right primitive ideal is defined similarly. Actually (despite the name) the left and right primitive ideals are always two-sided ideals. Primitive ideals are prime. A factor rings constructed with a right (left) primitive ideals is a right (left) primitive ring. For commutative rings the primitive ideals are maximal, and so commutative primitive rings are all fields.
• Irreducible ideal: An ideal is said to be irreducible if it cannot be written as an intersection of ideals which properly contain it.
• Comaximal ideals: Two ideals $\mathfrak{i}, \mathfrak{j}$ are said to be comaximal if x + y = 1 for some $x \in \mathfrak{i}$ and $y \in \mathfrak{j}$.

## Properties

• An ideal is proper if and only if it does not contain 1.
• The proper ideals can be partially ordered via subset inclusion and therefore as a consequence of Zorn's lemma every proper ideal is contained in a maximal ideal.
• Because zero belongs to it, any ideal is nonempty.
• The ring R can be considered as a left module over itself, and the left ideals of R are then seen as the submodules of this module. Similarly, the right ideals are submodules of R as a right module over itself, and the two-sided ideals are submodules of R as a bimodule over itself. If R is commutative, then all three sorts of module are the same, just as all three sorts of ideal are the same.
• Every ideal is a pseudo-ring.
• The ideals of a ring form a semiring under addition and multiplication of ideals.

## Ideal operations

The sum and product of ideals are defined as follows. For $\mathfrak{a}$ and $\mathfrak{b}$, ideals of a ring R, $\mathfrak{a}+\mathfrak{b}:=\{a+b \,|\, a \in \mathfrak{a} \mbox{ and } b \in \mathfrak{b}\}$

and $\mathfrak{a} \mathfrak{b}:=\{a_1b_1+ \dots + a_nb_n \,|\, a_i \in \mathfrak{a} \mbox{ and } b_i \in \mathfrak{b}, i=1, 2, \dots, n; \mbox{ for } n=1, 2, \dots\},$

i.e. the product of two ideals $\mathfrak{a}$ and $\mathfrak{b}$ is defined to be the ideal $\mathfrak{a}\mathfrak{b}$ generated by all products of the form ab with a in $\mathfrak{a}$ and b in $\mathfrak{b}$. The product $\mathfrak{a}\mathfrak{b}$ is contained in the intersection of $\mathfrak{a}$ and $\mathfrak{b}$.

The sum and the intersection of ideals is again an ideal; with these two operations as join and meet, the set of all ideals of a given ring forms a complete modular lattice. Also, the union of two ideals is a subset of the sum of those two ideals, because for any element a inside an ideal, we can write it as a+0, or 0+a, therefore, it is contained in the sum as well. However, the union of two ideals is not necessarily an ideal.

## Ideals and congruence relations

There is a bijective correspondence between ideals and congruence relations (equivalence relations that respect the ring structure) on the ring:

Given an ideal I of a ring R, let x ~ y if x-y ∈ I. Then ~ is a congruence relation on R.

Conversely, given a congruence relation ~ on R, let I = {x : x ~ 0}. Then I is an ideal of R.

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