Английская Википедия:Ideal (ring theory)

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In mathematics, and more specifically in ring theory, an ideal of a ring is a special subset of its elements. Ideals generalize certain subsets of the integers, such as the even numbers or the multiples of 3. Addition and subtraction of even numbers preserves evenness, and multiplying an even number by any integer (even or odd) results in an even number; these closure and absorption properties are the defining properties of an ideal. An ideal can be used to construct a quotient ring in a way similar to how, in group theory, a normal subgroup can be used to construct a quotient group.

Among the integers, the ideals correspond one-for-one with the non-negative integers: in this ring, every ideal is a principal ideal consisting of the multiples of a single non-negative number. However, in other rings, the ideals may not correspond directly to the ring elements, and certain properties of integers, when generalized to rings, attach more naturally to the ideals than to the elements of the ring. For instance, the prime ideals of a ring are analogous to prime numbers, and the Chinese remainder theorem can be generalized to ideals. There is a version of unique prime factorization for the ideals of a Dedekind domain (a type of ring important in number theory).

The related, but distinct, concept of an 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

Ernst Kummer invented the concept of ideal numbers to serve as the "missing" factors in number rings in which unique factorization fails; here the word "ideal" is in the sense of existing in imagination only, in analogy with "ideal" objects in geometry such as points at infinity.[1] In 1876, Richard Dedekind replaced Kummer's undefined concept by concrete sets of numbers, sets that he called ideals, in the third edition of Dirichlet's book Vorlesungen über Zahlentheorie, to which Dedekind had added many supplements.[1][2][3] Later the notion was extended beyond number rings to the setting of polynomial rings and other commutative rings by David Hilbert and especially Emmy Noether.

Definitions and motivation

For an arbitrary ring Шаблон:Tmath, let <math>(R,+)</math> be its additive group. A subset Шаблон:Mvar is called a left ideal of <math>R</math> if it is an additive subgroup of <math>R</math> that "absorbs multiplication from the left by elements of Шаблон:Tmath"; that is, <math>I</math> is a left ideal if it satisfies the following two conditions:

  1. <math>(I,+)</math> is a subgroup of Шаблон:Tmath,
  2. For every <math>r \in R</math> and every Шаблон:Tmath, the product <math>r x</math> is in Шаблон:Tmath.

A right ideal is defined with the condition <math>rx\in I</math> replaced by Шаблон:Tmath. A two-sided ideal is a left ideal that is also a right ideal, and is sometimes simply called an ideal. In the language of modules, the definitions mean that a left (resp. right, two-sided) ideal of <math>R</math> is an <math>R</math>-submodule of <math>R</math> when <math>R</math> is viewed as a left (resp. right, bi-) <math>R</math>-module. When <math>R</math> is a commutative ring, the definitions of left, right, and two-sided ideal coincide, and the term ideal is used alone.

To understand the concept of an ideal, consider how ideals arise in the construction of rings of "elements modulo". For concreteness, let us look at the ring <math>\Z/n\Z</math> of integers modulo <math>n</math> given an integer <math>n\in\Z</math> (Шаблон:Tmath is a commutative ring). The key observation here is that we obtain <math>\Z/n\Z</math> by taking the integer line <math>\Z</math> and wrapping it around itself so that various integers get identified. In doing so, we must satisfy two requirements:

  1. <math>n</math> must be identified with 0 since <math>n</math> is congruent to 0 modulo Шаблон:Tmath.
  2. the resulting structure must again be a ring.

The second requirement forces us to make additional identifications (i.e., it determines the precise way in which we must wrap <math>\Z</math> around itself). The notion of an ideal arises when we ask the question:

What is the exact set of integers that we are forced to identify with 0?

The answer is, unsurprisingly, the set <math>n\Z=\{nm\mid m\in\Z\}</math> of all integers congruent to 0 modulo <math>n</math>. That is, we must wrap <math>\Z</math> around itself infinitely many times so that the integers <math>\ldots,-2n,-n,n,2n,3n,\ldots</math> will all align with 0. If we look at what properties this set must satisfy in order to ensure that <math>\Z/n\Z</math> is a ring, then we arrive at the definition of an ideal. Indeed, one can directly verify that <math>n\Z</math> is an ideal of Шаблон:Tmath.

Remark. Identifications with elements other than 0 also need to be made. For example, the elements in <math>1+n\Z</math> must be identified with 1, the elements in <math>2+n\Z</math> must be identified with 2, and so on. Those, however, are uniquely determined by <math>n\Z</math> since <math>\Z</math> is an additive group.

We can make a similar construction in any commutative ring Шаблон:Tmath: start with an arbitrary Шаблон:Tmath, and then identify with 0 all elements of the ideal Шаблон:Tmath. It turns out that the ideal <math>xR</math> is the smallest ideal that contains Шаблон:Tmath, called the ideal generated by <math>x</math>. More generally, we can start with an arbitrary subset Шаблон:Tmath, and then identify with 0 all the elements in the ideal generated by Шаблон:Tmath: the smallest ideal <math>(S)</math> such that Шаблон:Tmath. The ring that we obtain after the identification depends only on the ideal <math>(S)</math> and not on the set <math>S</math> that we started with. That is, if Шаблон:Tmath, then the resulting rings will be the same.

Therefore, an ideal <math>I</math> of a commutative ring <math>R</math> captures canonically the information needed to obtain the ring of elements of <math>R</math> modulo a given subset Шаблон:Tmath. The elements of Шаблон:Tmath, by definition, are those that are congruent to zero, that is, identified with zero in the resulting ring. The resulting ring is called the quotient of <math>R</math> by <math>I</math> and is denoted Шаблон:Tmath. Intuitively, the definition of an ideal postulates two natural conditions necessary for <math>I</math> to contain all elements designated as "zeros" by Шаблон:Tmath:

  1. <math>I</math> is an additive subgroup of Шаблон:Tmath: the zero 0 of <math>R</math> is a "zero" Шаблон:Tmath, and if <math>x_1\in I</math> and <math>x_2\in I</math> are "zeros", then <math>x_1-x_2\in I</math> is a "zero" too.
  2. Any <math>r\in R</math> multiplied by a "zero" <math>x\in I</math> is a "zero" Шаблон:Tmath.

It turns out that the above conditions are also sufficient for <math>I</math> to contain all the necessary "zeros": no other elements have to be designated as "zero" in order to form Шаблон:Tmath. (In fact, no other elements should be designated as "zero" if we want to make the fewest identifications.)

Remark. The above construction still works using two-sided ideals even if <math>R</math> is not necessarily commutative.

Examples and properties

(For the sake of brevity, some results are stated only for left ideals but are usually also true for right ideals with appropriate notation changes.)

  • In a ring R, the set R itself forms a two-sided ideal of R called the unit ideal. It is often also denoted by <math>(1)</math> since it is precisely the two-sided ideal generated (see below) by the unity Шаблон:Tmath. Also, the set <math>\{ 0_R \}</math> consisting of only the additive identity 0R forms a two-sided ideal called the zero ideal and is denoted by Шаблон:Tmath.[note 1] Every (left, right or two-sided) ideal contains the zero ideal and is contained in the unit ideal.Шаблон:Sfnp
  • An (left, right or two-sided) ideal that is not the unit ideal is called a proper ideal (as it is a proper subset).[4] Note: a left ideal <math>\mathfrak{a}</math> is proper if and only if it does not contain a unit element, since if <math>u \in \mathfrak{a}</math> is a unit element, then <math>r = (r u^{-1}) u \in \mathfrak{a}</math> for every Шаблон:Tmath. Typically there are plenty of proper ideals. In fact, if R is a skew-field, then <math>(0), (1)</math> are its only ideals and conversely: that is, a nonzero ring R is a skew-field if <math>(0), (1)</math> are the only left (or right) ideals. (Proof: if <math>x</math> is a nonzero element, then the principal left ideal <math>Rx</math> (see below) is nonzero and thus <math>Rx = (1)</math>; i.e., <math>yx = 1</math> for some nonzero Шаблон:Tmath. Likewise, <math>zy = 1</math> for some nonzero <math>z</math>. Then <math>z = z(yx) = (zy)x = x</math>.)
  • The even integers form an ideal in the ring <math>\mathbb{Z}</math> of all integers, since the sum of any two even integers is even, and the product of any integer with an even integer is also even; this ideal is usually denoted by Шаблон:Tmath. More generally, the set of all integers divisible by a fixed integer <math>n</math> is an ideal denoted Шаблон:Tmath. In fact, every non-zero ideal of the ring <math>\mathbb{Z}</math> is generated by its smallest positive element, as a consequence of Euclidean division, so <math>\mathbb{Z}</math> is a principal ideal domain.Шаблон:Sfnp
  • The set of all polynomials with real coefficients that are divisible by the polynomial <math>x^2+1</math> is an ideal in the ring of all real-coefficient polynomials Шаблон:Tmath.
  • Take a ring <math>R</math> and positive integer Шаблон:Tmath. For each Шаблон:Tmath, the set of all <math>n\times n</math> matrices with entries in <math>R</math> whose <math>i</math>-th row is zero is a right ideal in the ring <math>M_n(R)</math> of all <math>n\times n</math> matrices with entries in Шаблон:Tmath. It is not a left ideal. Similarly, for each Шаблон:Tmath, the set of all <math>n\times n</math> matrices whose <math>j</math>-th column is zero is a left ideal but not a right ideal.
  • The ring <math>C(\mathbb{R})</math> of all continuous functions <math>f</math> from <math>\mathbb{R}</math> to <math>\mathbb{R}</math> under pointwise multiplication contains the ideal of all continuous functions <math>f</math> such that Шаблон:Tmath.Шаблон:Sfnp Another ideal in <math>C(\mathbb{R})</math> is given by those functions that vanish for large enough arguments, i.e. those continuous functions <math>f</math> for which there exists a number <math>L>0</math> such that <math>f(x)=0</math> whenever Шаблон:Tmath.
  • A ring is called a simple ring if it is nonzero and has no two-sided ideals other than Шаблон:Tmath. Thus, a skew-field is simple and a simple commutative ring is a field. The matrix ring over a skew-field is a simple ring.
  • If <math>f: R \to S</math> is a ring homomorphism, then the kernel <math>\ker(f) = f^{-1}(0_S)</math> is a two-sided ideal of Шаблон:Tmath.Шаблон:Sfnp By definition, Шаблон:Tmath, and thus if <math>S</math> is not the zero ring (so Шаблон:Tmath), then <math>\ker(f)</math> is a proper ideal. More generally, for each left ideal I of S, the pre-image <math>f^{-1}(I)</math> is a left ideal. If I is a left ideal of R, then <math>f(I)</math> is a left ideal of the subring <math>f(R)</math> of S: unless f is surjective, <math>f(I)</math> need not be an ideal of S; see also #Extension and contraction of an ideal below.
  • Ideal correspondence: Given a surjective ring homomorphism Шаблон:Tmath, there is a bijective order-preserving correspondence between the left (resp. right, two-sided) ideals of <math>R</math> containing the kernel of <math>f</math> and the left (resp. right, two-sided) ideals of <math>S</math>: the correspondence is given by <math>I \mapsto f(I)</math> and the pre-image Шаблон:Tmath. Moreover, for commutative rings, this bijective correspondence restricts to prime ideals, maximal ideals, and radical ideals (see the Types of ideals section for the definitions of these ideals).
  • (For those who know modules) If M is a left R-module and <math>S \subset M</math> a subset, then the annihilator <math>\operatorname{Ann}_R(S) = \{ r \in R \mid rs = 0, s \in S \}</math> of S is a left ideal. Given ideals <math>\mathfrak{a}, \mathfrak{b}</math> of a commutative ring R, the R-annihilator of <math>(\mathfrak{b} + \mathfrak{a})/\mathfrak{a}</math> is an ideal of R called the ideal quotient of <math>\mathfrak{a}</math> by <math>\mathfrak{b}</math> and is denoted by Шаблон:Tmath; it is an instance of idealizer in commutative algebra.
  • Let <math>\mathfrak{a}_i, i \in S</math> be an ascending chain of left ideals in a ring R; i.e., <math>S</math> is a totally ordered set and <math>\mathfrak{a}_i \subset \mathfrak{a}_j</math> for each Шаблон:Tmath. Then the union <math>\textstyle \bigcup_{i \in S} \mathfrak{a}_i</math> is a left ideal of R. (Note: this fact remains true even if R is without the unity 1.)
  • The above fact together with Zorn's lemma proves the following: if <math>E \subset R</math> is a possibly empty subset and <math>\mathfrak{a}_0 \subset R</math> is a left ideal that is disjoint from E, then there is an ideal that is maximal among the ideals containing <math>\mathfrak{a}_0</math> and disjoint from E. (Again this is still valid if the ring R lacks the unity 1.) When <math>R \ne 0</math>, taking <math>\mathfrak{a}_0 = (0)</math> and Шаблон:Tmath, in particular, there exists a left ideal that is maximal among proper left ideals (often simply called a maximal left ideal); see Krull's theorem for more.
  • An arbitrary union of ideals need not be an ideal, but the following is still true: given a possibly empty subset X of R, there is the smallest left ideal containing X, called the left ideal generated by X and is denoted by Шаблон:Tmath. Such an ideal exists since it is the intersection of all left ideals containing X. Equivalently, <math>RX</math> is the set of all the (finite) left R-linear combinations of elements of X over R:
    <math>RX = \{r_1x_1+\dots+r_nx_n \mid n\in\mathbb{N}, r_i\in R, x_i\in X\}.</math>
(since such a span is the smallest left ideal containing X.)[note 2] A right (resp. two-sided) ideal generated by X is defined in the similar way. For "two-sided", one has to use linear combinations from both sides; i.e.,
<math>RXR = \{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\}.\,</math>

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.

Two other important terms using "ideal" are not always ideals of their ring. See their respective articles for details:

  • Fractional ideal: This is usually defined when R is a commutative domain with quotient field K. Despite their names, fractional ideals are R submodules of K with a special property. If the fractional ideal is contained entirely in R, then it is truly an ideal of R.
  • Invertible ideal: Usually an invertible ideal A is defined as a fractional ideal for which there is another fractional ideal B such that Шаблон:Math. Some authors may also apply "invertible ideal" to ordinary ring ideals A and B with Шаблон:Math in rings other than domains.

Ideal operations

The sum and product of ideals are defined as follows. For <math>\mathfrak{a}</math> and Шаблон:Tmath, left (resp. right) ideals of a ring R, their sum is

<math>\mathfrak{a}+\mathfrak{b}:=\{a+b \mid a \in \mathfrak{a} \mbox{ and } b \in \mathfrak{b}\}</math>,

which is a left (resp. right) ideal, and, if <math>\mathfrak{a}, \mathfrak{b}</math> are two-sided,

<math>\mathfrak{a} \mathfrak{b}:=\{a_1b_1+ \dots + a_nb_n \mid a_i \in \mathfrak{a} \mbox{ and } b_i \in \mathfrak{b}, i=1, 2, \dots, n; \mbox{ for } n=1, 2, \dots\},</math>

i.e. the product is the ideal generated by all products of the form ab with a in <math>\mathfrak{a}</math> and b in Шаблон:Tmath.

Note <math>\mathfrak{a} + \mathfrak{b}</math> is the smallest left (resp. right) ideal containing both <math>\mathfrak{a}</math> and <math>\mathfrak{b}</math> (or the union Шаблон:Tmath), while the product <math>\mathfrak{a}\mathfrak{b}</math> is contained in the intersection of <math>\mathfrak{a}</math> and Шаблон:Tmath.

The distributive law holds for two-sided ideals Шаблон:Tmath,

If a product is replaced by an intersection, a partial distributive law holds:

<math>\mathfrak{a} \cap (\mathfrak{b} + \mathfrak{c}) \supset \mathfrak{a} \cap \mathfrak{b} + \mathfrak{a} \cap \mathfrak{c}</math>

where the equality holds if <math>\mathfrak{a}</math> contains <math>\mathfrak{b}</math> or <math>\mathfrak{c}</math>.

Remark: 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. The lattice is not, in general, a distributive lattice. The three operations of intersection, sum (or join), and product make the set of ideals of a commutative ring into a quantale.

If <math>\mathfrak{a}, \mathfrak{b}</math> are ideals of a commutative ring R, then <math>\mathfrak{a} \cap \mathfrak{b} = \mathfrak{a} \mathfrak{b}</math> in the following two cases (at least)

(More generally, the difference between a product and an intersection of ideals is measured by the Tor functor: Шаблон:Tmath.[7])

An integral domain is called a Dedekind domain if for each pair of ideals <math>\mathfrak{a} \subset \mathfrak{b}</math>, there is an ideal <math>\mathfrak{c}</math> such that Шаблон:Tmath.Шаблон:Sfnp It can then be shown that every nonzero ideal of a Dedekind domain can be uniquely written as a product of maximal ideals, a generalization of the fundamental theorem of arithmetic.

Examples of ideal operations

In <math>\mathbb{Z}</math> we have

<math>(n)\cap(m) = \operatorname{lcm}(n,m)\mathbb{Z}</math>

since <math>(n)\cap(m)</math> is the set of integers that are divisible by both <math>n</math> and Шаблон:Tmath.

Let <math>R = \mathbb{C}[x,y,z,w]</math> and let Шаблон:Tmath. Then,

  • <math> \mathfrak{a} + \mathfrak{b} = (z,w, x+z, y+w) = (x, y, z, w)</math> and <math>\mathfrak{a} + \mathfrak{c} = (z, w, x)</math>
  • <math>\mathfrak{a}\mathfrak{b} = (z(x + z), z(y + w), w(x + z), w(y + w))= (z^2 + xz, zy + wz, wx + wz, wy + w^2)</math>
  • <math>\mathfrak{a}\mathfrak{c} = (xz + z^2, zw, xw + zw, w^2)</math>
  • <math>\mathfrak{a} \cap \mathfrak{b} = \mathfrak{a}\mathfrak{b}</math> while <math>\mathfrak{a} \cap \mathfrak{c} = (w, xz + z^2) \neq \mathfrak{a}\mathfrak{c}</math>

In the first computation, we see the general pattern for taking the sum of two finitely generated ideals, it is the ideal generated by the union of their generators. In the last three we observe that products and intersections agree whenever the two ideals intersect in the zero ideal. These computations can be checked using Macaulay2.[8][9][10]

Radical of a ring

Шаблон:Main

Ideals appear naturally in the study of modules, especially in the form of a radical.

For simplicity, we work with commutative rings but, with some changes, the results are also true for non-commutative rings.

Let R be a commutative ring. By definition, a primitive ideal of R is the annihilator of a (nonzero) simple R-module. The Jacobson radical <math>J = \operatorname{Jac}(R)</math> of R is the intersection of all primitive ideals. Equivalently,

<math>J = \bigcap_{\mathfrak{m} \text{ maximal ideals}} \mathfrak{m}.</math>

Indeed, if <math>M</math> is a simple module and x is a nonzero element in M, then <math>Rx = M</math> and <math>R/\operatorname{Ann}(M) = R/\operatorname{Ann}(x) \simeq M</math>, meaning <math>\operatorname{Ann}(M)</math> is a maximal ideal. Conversely, if <math>\mathfrak{m}</math> is a maximal ideal, then <math>\mathfrak{m}</math> is the annihilator of the simple R-module Шаблон:Tmath. There is also another characterization (the proof is not hard):

<math>J = \{ x \in R \mid 1 - yx \, \text{ is a unit element for every } y \in R\}.</math>

For a not-necessarily-commutative ring, it is a general fact that <math>1 - yx</math> is a unit element if and only if <math>1 - xy</math> is (see the link) and so this last characterization shows that the radical can be defined both in terms of left and right primitive ideals.

The following simple but important fact (Nakayama's lemma) is built-in to the definition of a Jacobson radical: if M is a module such that Шаблон:Tmath, then M does not admit a maximal submodule, since if there is a maximal submodule Шаблон:Tmath, <math>J \cdot (M/L) = 0</math> and so Шаблон:Tmath, a contradiction. Since a nonzero finitely generated module admits a maximal submodule, in particular, one has:

If <math>JM = M</math> and M is finitely generated, then Шаблон:Tmath.

A maximal ideal is a prime ideal and so one has

<math>\operatorname{nil}(R) =

\bigcap_{\mathfrak{p} \text { prime ideals }} \mathfrak{p} \subset \operatorname{Jac}(R)</math> where the intersection on the left is called the nilradical of R. As it turns out, <math>\operatorname{nil}(R)</math> is also the set of nilpotent elements of R.

If R is an Artinian ring, then <math>\operatorname{Jac}(R)</math> is nilpotent and Шаблон:Tmath. (Proof: first note the DCC implies <math>J^n = J^{n+1}</math> for some n. If (DCC) <math>\mathfrak{a} \supsetneq \operatorname{Ann}(J^n)</math> is an ideal properly minimal over the latter, then <math>J \cdot (\mathfrak{a}/\operatorname{Ann}(J^n)) = 0</math>. That is, Шаблон:Tmath, a contradiction.)

Extension and contraction of an ideal

Let A and B be two commutative rings, and let f : A → B be a ring homomorphism. If <math>\mathfrak{a}</math> is an ideal in A, then <math>f(\mathfrak{a})</math> need not be an ideal in B (e.g. take f to be the inclusion of the ring of integers Z into the field of rationals Q). The extension <math>\mathfrak{a}^e</math> of <math>\mathfrak{a}</math> in B is defined to be the ideal in B generated by Шаблон:Tmath. Explicitly,

<math>\mathfrak{a}^e = \Big\{ \sum y_if(x_i) : x_i \in \mathfrak{a}, y_i \in B \Big\}</math>

If <math>\mathfrak{b}</math> is an ideal of B, then <math>f^{-1}(\mathfrak{b})</math> is always an ideal of A, called the contraction <math>\mathfrak{b}^c</math> of <math>\mathfrak{b}</math> to A.

Assuming f : A → B is a ring homomorphism, <math>\mathfrak{a}</math> is an ideal in A, <math>\mathfrak{b}</math> is an ideal in B, then:

  • <math>\mathfrak{b}</math> is prime in B <math>\Rightarrow</math> <math>\mathfrak{b}^c</math> is prime in A.
  • <math>\mathfrak{a}^{ec} \supseteq \mathfrak{a}</math>
  • <math>\mathfrak{b}^{ce} \subseteq \mathfrak{b}</math>

It is false, in general, that <math>\mathfrak{a}</math> being prime (or maximal) in A implies that <math>\mathfrak{a}^e</math> is prime (or maximal) in B. Many classic examples of this stem from algebraic number theory. For example, embedding Шаблон:Tmath. In <math>B = \mathbb{Z}\left\lbrack i \right\rbrack</math>, the element 2 factors as <math>2 = (1 + i)(1 - i)</math> where (one can show) neither of <math>1 + i, 1 - i</math> are units in B. So <math>(2)^e</math> is not prime in B (and therefore not maximal, as well). Indeed, <math>(1 \pm i)^2 = \pm 2i</math> shows that Шаблон:Tmath, Шаблон:Tmath, and therefore Шаблон:Tmath.

On the other hand, if f is surjective and <math> \mathfrak{a} \supseteq \ker f</math> then:

  • <math>\mathfrak{a}^{ec}=\mathfrak{a} </math> and Шаблон:Tmath.
  • <math>\mathfrak{a}</math> is a prime ideal in A <math>\Leftrightarrow</math> <math>\mathfrak{a}^e</math> is a prime ideal in B.
  • <math>\mathfrak{a}</math> is a maximal ideal in A <math>\Leftrightarrow</math> <math>\mathfrak{a}^e</math> is a maximal ideal in B.

Remark: Let K be a field extension of L, and let B and A be the rings of integers of K and L, respectively. Then B is an integral extension of A, and we let f be the inclusion map from A to B. The behaviour of a prime ideal <math>\mathfrak{a} = \mathfrak{p}</math> of A under extension is one of the central problems of algebraic number theory.

The following is sometimes useful:Шаблон:Sfnp a prime ideal <math>\mathfrak{p}</math> is a contraction of a prime ideal if and only if Шаблон:Tmath. (Proof: Assuming the latter, note <math>\mathfrak{p}^e B_{\mathfrak{p}} = B_{\mathfrak{p}} \Rightarrow \mathfrak{p}^e</math> intersects Шаблон:Tmath, a contradiction. Now, the prime ideals of <math>B_{\mathfrak{p}}</math> correspond to those in B that are disjoint from Шаблон:Tmath. Hence, there is a prime ideal <math>\mathfrak{q}</math> of B, disjoint from Шаблон:Tmath, such that <math>\mathfrak{q} B_{\mathfrak{p}}</math> is a maximal ideal containing Шаблон:Tmath. One then checks that <math>\mathfrak{q}</math> lies over Шаблон:Tmath. The converse is obvious.)

Generalizations

Ideals can be generalized to any monoid object Шаблон:Tmath, where <math>R</math> is the object where the monoid structure has been forgotten. A left ideal of <math>R</math> is a subobject <math>I</math> that "absorbs multiplication from the left by elements of Шаблон:Tmath"; that is, <math>I</math> is a left ideal if it satisfies the following two conditions:

  1. <math>I</math> is a subobject of <math>R</math>
  2. For every <math>r \in (R,\otimes)</math> and every Шаблон:Tmath, the product <math>r \otimes x</math> is in Шаблон:Tmath.

A right ideal is defined with the condition "Шаблон:Tmath" replaced by "'Шаблон:Tmath". A two-sided ideal is a left ideal that is also a right ideal, and is sometimes simply called an ideal. When <math>R</math> is a commutative monoid object respectively, the definitions of left, right, and two-sided ideal coincide, and the term ideal is used alone.

An ideal can also be thought of as a specific type of [[Module_(mathematics)|Шаблон:Math-module]]. If we consider <math>R</math> as a left <math>R</math>-module (by left multiplication), then a left ideal <math>I</math> is really just a left sub-module of Шаблон:Tmath. In other words, <math>I</math> is a left (right) ideal of <math>R</math> if and only if it is a left (right) <math>R</math>-module that is a subset of Шаблон:Tmath. <math>I</math> is a two-sided ideal if it is a sub-<math>R</math>-bimodule of Шаблон:Tmath.

Example: If we let Шаблон:Tmath, an ideal of <math>\mathbb{Z}</math> is an abelian group that is a subset of Шаблон:Tmath, i.e. <math>m\mathbb{Z}</math> for some Шаблон:Tmath. So these give all the ideals of Шаблон:Tmath.

See also

Notes

Шаблон:Reflist

References

Шаблон:Reflist Шаблон:Refbegin

Шаблон:Refend

External links

Шаблон:Authority control

Партнерские ресурсы
Криптовалюты
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Хостинг
Разное

  1. ↑ 1,0 1,1 Шаблон:Cite book
  2. ↑ Шаблон:Cite book
  3. ↑ Шаблон:Cite book
  4. ↑ Шаблон:Harvnb
  5. ↑ Because simple commutative rings are fields. See Шаблон:Cite book
  6. ↑ Шаблон:Cite book
  7. ↑ Шаблон:Harvnb
  8. ↑ Шаблон:Cite web
  9. ↑ Шаблон:Cite web
  10. ↑ Шаблон:Cite web


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