In ring theory, a branch of abstract algebra, 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 other integer results in another 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.
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.
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 $(R,+,\backslash cdot)$, let $(R,+)$ be its additive group. A subset $I$ is called a left ideal of $R$ if it is an additive subgroup of $R$ that "absorbs multiplication from the left by elements of $R$"; that is, $I$ is a left ideal if it satisfies the following two conditions:
# $(I,+)$ is a subgroup of $(R,+),$
# For every $r\; \backslash in\; R$ and every $x\; \backslash in\; I$, the product $r\; x$ is in $I$.
A right ideal is defined with the condition replaced by . 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 ''R'' is precisely a left (resp. right, bi-) ''R''-submodule of ''R'' when ''R'' is viewed as an ''R''-module. When ''R'' 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 ℤ_{''n''} of integers modulo a given integer ''n'' ∈ ℤ (note that ℤ is a commutative ring). The key observation here is that we obtain ℤ_{''n''} by taking the integer line ℤ and wrapping it around itself so that various integers get identified. In doing so, we must satisfy two requirements: 1) ''n'' must be identified with 0 since ''n'' is congruent to 0 modulo ''n'', and 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 ℤ around itself). The notion of an ideal arises when we ask the question: _{''n''} is a ring, then we arrive at the definition of an ideal. Indeed, one can directly verify that ''n''ℤ is an ideal of ℤ.
Remark. Identifications with elements other than 0 also need to be made. For example, the elements in must be identified with 1, the elements in must be identified with 2, and so on. Those, however, are uniquely determined by ''n''ℤ since ℤ is an additive group.
We can make a similar construction in any commutative ring ''R'': start with an arbitrary , and then identify with 0 all elements of the ideal It turns out that the ideal ''xR'' is the smallest ideal that contains ''x'', called the ideal generated by ''x''. More generally, we can start with an arbitrary subset , and then identify with 0 all the elements in the ideal generated by ''S'': the smallest ideal (''S'') such that . The ring that we obtain after the identification depends only on the ideal (''S'') and not on the set ''S'' that we started with. That is, if , then the resulting rings will be the same.
Therefore, an ideal ''I'' of a commutative ring ''R'' captures canonically the information needed to obtain the ring of elements of ''R'' modulo a given subset . The elements of ''I'', 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 ''R'' by ''I'' and is denoted ''R''/''I''. Intuitively, the definition of an ideal postulates two natural conditions necessary for ''I'' to contain all elements designated as "zeros" by ''R''/''I'':
# ''I'' is an additive subgroup of ''R'': the zero 0 of ''R'' is a "zero" , and if and are "zeros", then is a "zero" too.
# Any multiplied by a "zero" is a "zero" .
It turns out that the above conditions are also sufficient for ''I'' to contain all the necessary "zeros": no other elements have to be designated as "zero" in order to form ''R''/''I''. (In fact, no other elements should be designated as "zero" if we want to make the fewest identifications.)
Remark. If ''R'' is not necessarily commutative, the above construction still works using two-sided ideals.

** 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 $(1)$ since it is precisely the two-sided ideal generated (see below) by the unity $1\_R$. Also, the set $\backslash $ consisting of only the additive identity 0_{''R''} forms a two-sided ideal called the zero ideal and is denoted by $(0)$.Some authors call the zero and unit ideals of a ring ''R'' the trivial ideals of ''R''. Every (left, right or two-sided) ideal contains the zero ideal and is contained in the unit ideal.
* An (left, right or two-sided) ideal that is not the unit ideal is called a proper ideal (as it is a proper subset). Note: a left ideal $\backslash mathfrak$ is proper if and only if it does not contain a unit element, since if $u\; \backslash in\; \backslash mathfrak$ is a unit element, then $r\; =\; (r\; u^)\; u\; \backslash in\; \backslash mathfrak$ for every $r\; \backslash in\; R$. Typically there are plenty of proper ideals. In fact, if ''R'' is a skew-field, then $(0),\; (1)$ are its only ideals and conversely: that is, a nonzero ring ''R'' is a skew-field if $(0),\; (1)$ are the only left (or right) ideals. (Proof: if $x$ is a nonzero element, then the principal left ideal $Rx$ (see below) is nonzero and thus $Rx\; =\; (1)$; i.e., $yx\; =\; 1$ for some nonzero $y$. Likewise, $zy\; =\; 1$ for some nonzero $z$. Then $z\; =\; z(yx)\; =\; (zy)x\; =\; x$.)
* The even integers form an ideal in the ring $\backslash mathbb$ of all integers; it is usually denoted by $2\backslash mathbb$. 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 $n\backslash mathbb$.
* The set of all polynomials with real coefficients which are divisible by the polynomial ''x''^{2} + 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 $C(\backslash mathbb)$ of all continuous functions ''f'' from $\backslash mathbb$ to $\backslash mathbb$ under pointwise multiplication contains the ideal of all continuous functions ''f'' such that ''f''(1) = 0. Another ideal in $C(\backslash mathbb)$ 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 > ''L''.
* A ring is called a simple ring if it is nonzero and has no two-sided ideals other than $(0),\; (1)$. 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 $f:\; R\; \backslash to\; S$ is a ring homomorphism, then the kernel $\backslash ker(f)\; =\; f^(0\_S)$ is a two-sided ideal of $R$. By definition, $f(1\_R)\; =\; 1\_S$, and thus if $S$ is not the zero ring (so $1\_S\backslash ne0\_S$), then $\backslash ker(f)$ is a proper ideal. More generally, for each left ideal ''I'' of ''S'', the pre-image $f^(I)$ is a left ideal. If ''I'' is a left ideal of ''R'', then $f(I)$ is a left ideal of the subring $f(R)$ of ''S'': unless ''f'' is surjective, $f(I)$ need not be an ideal of ''S''; see also #Extension and contraction of an ideal below.
* Ideal correspondence: Given a surjective ring homomorphism $f:\; R\; \backslash to\; S$, there is a bijective order-preserving correspondence between the left (resp. right, two-sided) ideals of $R$ containing the kernel of $f$ and the left (resp. right, two-sided) ideals of $S$: the correspondence is given by $I\; \backslash mapsto\; f(I)$ and the pre-image $J\; \backslash mapsto\; f^(J)$. 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 $S\; \backslash subset\; M$ a subset, then the annihilator $\backslash operatorname\_R(S)\; =\; \backslash $ of ''S'' is a left ideal. Given ideals $\backslash mathfrak,\; \backslash mathfrak$ of a commutative ring ''R'', the ''R''-annihilator of $(\backslash mathfrak\; +\; \backslash mathfrak)/\backslash mathfrak$ is an ideal of ''R'' called the ideal quotient of $\backslash mathfrak$ by $\backslash mathfrak$ and is denoted by $(\backslash mathfrak\; :\; \backslash mathfrak)$; it is an instance of idealizer in commutative algebra.
* Let $\backslash mathfrak\_i,\; i\; \backslash in\; S$ be an ascending chain of left ideals in a ring ''R''; i.e., $S$ is a totally ordered set and $\backslash mathfrak\_i\; \backslash subset\; \backslash mathfrak\_j$ for each $i\; <\; j$. Then the union $\backslash textstyle\; \backslash bigcup\_\; \backslash mathfrak\_i$ 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 $E\; \backslash subset\; R$ is a possibly empty subset and $\backslash mathfrak\_0\; \backslash subset\; R$ is a left ideal that is disjoint from ''E'', then there is an ideal that is maximal among the ideals containing $\backslash mathfrak\_0$ and disjoint from ''E''. (Again this is still valid if the ring ''R'' lacks the unity 1.) When $R\; \backslash ne\; 0$, taking $\backslash mathfrak\_0\; =\; (0)$ and $E\; =\; \backslash $, 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 $RX$. Such an ideal exists since it is the intersection of all left ideals containing ''X''. Equivalently, $RX$ is the set of all the (finite) left ''R''-linear combinations of elements of ''X'' over ''R'':
*:$RX\; =\; \backslash .$
:(since such a span is the smallest left ideal containing ''X''.)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 , and ''n''-fold sums of the form for every ''x'' in ''X'' and every ''n'' in the natural numbers. When ''R'' has a unit, this extra requirement becomes superfluous. 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.,
::$RXR\; =\; \backslash .\backslash ,$
*A left (resp. right, two-sided) ideal generated by a single element ''x'' is called the principal left (resp. right, two-sided) ideal generated by ''x'' and is denoted by $Rx$ (resp. $xR,\; RxR$). The principal two-sided ideal $RxR$ is often also denoted by $(x)$. If $X\; =\; \backslash $ is a finite set, then $RXR$ is also written as $(x\_1,\; \backslash dots,\; x\_n)$.
* In the ring $\backslash mathbb$ of integers, every ideal can be generated by a single number (so $\backslash mathbb$ is a principal ideal domain), as a consequence of Euclidean division (or some other way).
*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 if . Then ~ is a congruence relation on ''R''. Conversely, given a congruence relation ~ on ''R'', let . Then ''I'' is an ideal of ''R''.

** 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 proper subset of ''J''. The factor ring of a maximal ideal is a simple ring in general and is a field for commutative rings.
* Minimal ideal: A nonzero ideal is called minimal if it contains no other nonzero ideal.
* 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 ''a''^{''n''} 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 ''b''^{''n''} 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.
* Finitely generated ideal: This type of ideal is finitely generated as a module.
* Primitive ideal: A left primitive ideal is the annihilator of a simple left module.
* 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 $\backslash mathfrak,\; \backslash mathfrak$ are said to be comaximal if $x\; +\; y\; =\; 1$ for some $x\; \backslash in\; \backslash mathfrak$ and $y\; \backslash in\; \backslash mathfrak$.
* Regular ideal: This term has multiple uses. See the article for a list.
* Nil ideal: An ideal is a nil ideal if each of its elements is nilpotent.
* Nilpotent ideal: Some power of it is zero.
* Parameter ideal: an ideal generated by a system of parameters.
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 ''AB''=''BA''=''R''. Some authors may also apply "invertible ideal" to ordinary ring ideals ''A'' and ''B'' with ''AB''=''BA''=''R'' in rings other than domains.

** Ideal operations **

The sum and product of ideals are defined as follows. For $\backslash mathfrak$ and $\backslash mathfrak$, left (resp. right) ideals of a ring ''R'', their sum is
:$\backslash mathfrak+\backslash mathfrak:=\backslash $,
which is a left (resp. right) ideal,
and, if $\backslash mathfrak,\; \backslash mathfrak$ are two-sided,
:$\backslash mathfrak\; \backslash mathfrak:=\backslash ,$
i.e. the product is the ideal generated by all products of the form ''ab'' with ''a'' in $\backslash mathfrak$ and ''b'' in $\backslash mathfrak$.
Note $\backslash mathfrak\; +\; \backslash mathfrak$ is the smallest left (resp. right) ideal containing both $\backslash mathfrak$ and $\backslash mathfrak$ (or the union $\backslash mathfrak\; \backslash cup\; \backslash mathfrak$), while the product $\backslash mathfrak\backslash mathfrak$ is contained in the intersection of $\backslash mathfrak$ and $\backslash mathfrak$.
The distributive law holds for two-sided ideals $\backslash mathfrak,\; \backslash mathfrak,\; \backslash mathfrak$,
*$\backslash mathfrak(\backslash mathfrak\; +\; \backslash mathfrak)\; =\; \backslash mathfrak\; \backslash mathfrak\; +\; \backslash mathfrak\; \backslash mathfrak$,
*$(\backslash mathfrak\; +\; \backslash mathfrak)\; \backslash mathfrak\; =\; \backslash mathfrak\backslash mathfrak\; +\; \backslash mathfrak\backslash mathfrak$.
If a product is replaced by an intersection, a partial distributive law holds:
:$\backslash mathfrak\; \backslash cap\; (\backslash mathfrak\; +\; \backslash mathfrak)\; \backslash supset\; \backslash mathfrak\; \backslash cap\; \backslash mathfrak\; +\; \backslash mathfrak\; \backslash cap\; \backslash mathfrak$
where the equality holds if $\backslash mathfrak$ contains $\backslash mathfrak$ or $\backslash mathfrak$.
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 $\backslash mathfrak,\; \backslash mathfrak$ are ideals of a commutative ring ''R'', then $\backslash mathfrak\; \backslash cap\; \backslash mathfrak\; =\; \backslash mathfrak\; \backslash mathfrak$ in the following two cases (at least)
*$\backslash mathfrak\; +\; \backslash mathfrak\; =\; (1)$
*$\backslash mathfrak$ is generated by elements that form a regular sequence modulo $\backslash mathfrak$.
(More generally, the difference between a product and an intersection of ideals is measured by the Tor functor: $\backslash operatorname^R\_1(R/\backslash mathfrak,\; R/\backslash mathfrak)\; =\; (\backslash mathfrak\; \backslash cap\; \backslash mathfrak)/\; \backslash mathfrak\; \backslash mathfrak.$)
An integral domain is called a Dedekind domain if for each pair of ideals $\backslash mathfrak\; \backslash subset\; \backslash mathfrak$, there is an ideal $\backslash mathfrak$ such that $\backslash mathfrak\; \backslash mathfrak\; =\; \backslash mathfrak\; \backslash mathfrak$. 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 $\backslash mathbb$ we have
:$(n)\backslash cap(m)\; =\; \backslash operatorname(n,m)\backslash mathbb$
since $(n)\backslash cap(m)$ is the set of integers which are divisible by both $n$ and $m$.
Let $R\; =\; \backslash mathbb,y,z,w/math>\; and\; let$ I\; =\; (z,\; w),\backslash textJ\; =\; (x+z,y+w),\backslash textK\; =\; (x+z,\; w)$.\; Then,\; *$ I\; +\; J\; =\; (z,w,\; x+z,\; y+w)\; =\; (x,\; y,\; z,\; w)$and$ I\; +\; K\; =\; (z,\; w,\; x\; +\; z)$*$ IJ\; =\; (z(x\; +\; z),\; z(y\; +\; w),\; w(x\; +\; z),\; w(y\; +\; w))=\; (z^2\; +\; xz,\; zy\; +\; wz,\; wx\; +\; wz,\; wy\; +\; w^2)$*$ IK\; =\; (xz\; +\; z^2,\; zw,\; xw\; +\; zw,\; w^2)$*$ I\; \backslash cap\; J\; =\; IJ$while$ I\; \backslash cap\; K\; =\; (w,\; xz\; +\; z^2)\; \backslash neq\; IK$In\; the\; first\; computation,\; we\; see\; the\; general\; pattern\; for\; taking\; the\; sum\; of\; two\; finitely\; generated\; ideals,\; it\; is\; the\; ideal\; generated\; by\; the\; union\; of\; their\; generators.\; In\; the\; last\; three\; we\; observe\; that\; products\; and\; intersections\; agree\; whenever\; the\; two\; ideals\; intersect\; in\; the\; zero\; ideal.\; These\; computations\; can\; be\; checked\; usingMacaulay2.$

** Radical of a ring **

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 $J\; =\; \backslash operatorname(R)$ of ''R'' is the intersection of all primitive ideals. Equivalently,
:$J\; =\; \backslash bigcap\_\; \backslash mathfrak.$
Indeed, if $M$ is a simple module and ''x'' is a nonzero element in ''M'', then $Rx\; =\; M$ and $R/\backslash operatorname(M)\; =\; R/\backslash operatorname(x)\; \backslash simeq\; M$, meaning $\backslash operatorname(M)$ is a maximal ideal. Conversely, if $\backslash mathfrak$ is a maximal ideal, then $\backslash mathfrak$ is the annihilator of the simple ''R''-module $R/\backslash mathfrak$. There is also another characterization (the proof is not hard):
:$J\; =\; \backslash .$
For a not-necessarily-commutative ring, it is a general fact that $1\; -\; yx$ is a unit element if and only if $1\; -\; xy$ 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 $JM\; =\; M$, then ''M'' does not admit a maximal submodule, since if there is a maximal submodule $L\; \backslash subsetneq\; M$, $J\; \backslash cdot\; (M/L)\; =\; 0$ and so $M\; =\; JM\; \backslash subset\; L\; \backslash subsetneq\; M$, a contradiction. Since a nonzero finitely generated module admits a maximal submodule, in particular, one has:
:If $JM\; =\; M$ and ''M'' is finitely generated, then $M\; =\; 0.$
A maximal ideal is a prime ideal and so one has
:$\backslash operatorname(R)\; =\; \backslash bigcap\_\; \backslash mathfrak\; \backslash subset\; \backslash operatorname(R)$
where the intersection on the left is called the nilradical of ''R''. As it turns out, $\backslash operatorname(R)$ is also the set of nilpotent elements of ''R''.
If ''R'' is an Artinian ring, then $\backslash operatorname(R)$ is nilpotent and $\backslash operatorname(R)\; =\; \backslash operatorname(R)$. (Proof: first note the DCC implies $J^n\; =\; J^$ for some ''n''. If (DCC) $\backslash mathfrak\; \backslash supsetneq\; \backslash operatorname(J^n)$ is an ideal properly minimal over the latter, then $J\; \backslash cdot\; (\backslash mathfrak/\backslash operatorname(J^n))\; =\; 0$. That is, $J^n\; \backslash mathfrak\; =\; J^\; \backslash mathfrak\; =\; 0$, 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 $\backslash mathfrak$ is an ideal in ''A'', then $f(\backslash mathfrak)$ 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 $\backslash mathfrak^e$ of $\backslash mathfrak$ in ''B'' is defined to be the ideal in ''B'' generated by $f(\backslash mathfrak)$. Explicitly,
:$\backslash mathfrak^e\; =\; \backslash Big\backslash $
If $\backslash mathfrak$ is an ideal of ''B'', then $f^(\backslash mathfrak)$ is always an ideal of ''A'', called the contraction $\backslash mathfrak^c$ of $\backslash mathfrak$ to ''A''.
Assuming ''f'' : ''A'' → ''B'' is a ring homomorphism, $\backslash mathfrak$ is an ideal in ''A'', $\backslash mathfrak$ is an ideal in ''B'', then:
* $\backslash mathfrak$ is prime in ''B'' $\backslash Rightarrow$ $\backslash mathfrak^c$ is prime in ''A''.
* $\backslash mathfrak^\; \backslash supseteq\; \backslash mathfrak$
* $\backslash mathfrak^\; \backslash subseteq\; \backslash mathfrak$
It is false, in general, that $\backslash mathfrak$ being prime (or maximal) in ''A'' implies that $\backslash mathfrak^e$ is prime (or maximal) in ''B''. Many classic examples of this stem from algebraic number theory. For example, embedding $\backslash mathbb\; \backslash to\; \backslash mathbb\backslash left\backslash lbrack\; i\; \backslash right\backslash rbrack$. In $B\; =\; \backslash mathbb\backslash left\backslash lbrack\; i\; \backslash right\backslash rbrack$, the element 2 factors as $2\; =\; (1\; +\; i)(1\; -\; i)$ where (one can show) neither of $1\; +\; i,\; 1\; -\; i$ are units in ''B''. So $(2)^e$ is not prime in ''B'' (and therefore not maximal, as well). Indeed, $(1\; \backslash pm\; i)^2\; =\; \backslash pm\; 2i$ shows that $(1\; +\; i)\; =\; ((1\; -\; i)\; -\; (1\; -\; i)^2)$, $(1\; -\; i)\; =\; ((1\; +\; i)\; -\; (1\; +\; i)^2)$, and therefore $(2)^e\; =\; (1\; +\; i)^2$.
On the other hand, if ''f'' is surjective and $\backslash mathfrak\; \backslash supseteq\; \backslash ker\; f$ then:
* $\backslash mathfrak^=\backslash mathfrak$ and $\backslash mathfrak^=\backslash mathfrak$.
* $\backslash mathfrak$ is a prime ideal in ''A'' $\backslash Leftrightarrow$ $\backslash mathfrak^e$ is a prime ideal in ''B''.
* $\backslash mathfrak$ is a maximal ideal in ''A'' $\backslash Leftrightarrow$ $\backslash mathfrak^e$ 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 $\backslash mathfrak\; =\; \backslash mathfrak$ of ''A'' under extension is one of the central problems of algebraic number theory.
The following is sometimes useful: a prime ideal $\backslash mathfrak$ is a contraction of a prime ideal if and only if $\backslash mathfrak\; =\; \backslash mathfrak^$. (Proof: Assuming the latter, note $\backslash mathfrak^e\; B\_\; =\; B\_\; \backslash Rightarrow\; \backslash mathfrak^e$ intersects $A\; -\; \backslash mathfrak$, a contradiction. Now, the prime ideals of $B\_$ correspond to those in ''B'' that are disjoint from $A\; -\; \backslash mathfrak$. Hence, there is a prime ideal $\backslash mathfrak$ of ''B'', disjoint from $A\; -\; \backslash mathfrak$, such that $\backslash mathfrak\; B\_$ is a maximal ideal containing $\backslash mathfrak^e\; B\_$. One then checks that $\backslash mathfrak$ lies over $\backslash mathfrak$. The converse is obvious.)

** Generalisations **

Ideals can be generalised to any monoid object $(R,\backslash otimes)$, where $R$ is the object where the monoid structure has been forgotten. A left ideal of $R$ is a subobject $I$ that "absorbs multiplication from the left by elements of $R$"; that is, $I$ is a left ideal if it satisfies the following two conditions:
# $I$ is a subobject of $R$
# For every $r\; \backslash in\; (R,\backslash otimes)$ and every $x\; \backslash in\; (I,\; \backslash otimes)$, the product $r\; \backslash otimes\; x$ is in $(I,\; \backslash otimes)$.
A right ideal is defined with the condition "$r\; \backslash otimes\; x\; \backslash in\; (I,\; \backslash otimes)$" replaced by "'$x\; \backslash otimes\; r\; \backslash in\; (I,\; \backslash otimes)$". A two-sided ideal is a left ideal that is also a right ideal, and is sometimes simply called an ideal. When $R$ 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. If we consider $R$ as a left $R$-module (by left multiplication), then a left ideal $I$ is really just a left sub-module of $R$. In other words, $I$ is a left (right) ideal of $R$ if and only if it is a left (right) $R$-module which is a subset of $R$. $I$ is a two-sided ideal if it is a sub-$R$-bimodule of $R$.
Example: If we let $R=\backslash mathbb$, an ideal of $\backslash mathbb$ is an abelian group which is a subset of $\backslash mathbb$, i.e. $m\backslash mathbb$ for some $m\backslash in\backslash mathbb$. So these give all the ideals of $\backslash mathbb$.

** See also **

* Modular arithmetic
* Noether isomorphism theorem
* Boolean prime ideal theorem
* Ideal theory
* Ideal (order theory)
* Ideal norm
* Splitting of prime ideals in Galois extensions
* Ideal sheaf

** Notes **

** References **

*Atiyah, M. F. and Macdonald, I. G., ''Introduction to Commutative Algebra'', Perseus Books, 1969,
*
* Michiel Hazewinkel, Nadiya Gubareni, Nadezhda Mikhaĭlovna Gubareni, Vladimir V. Kirichenko. ''Algebras, rings and modules''. Volume 1. 2004. Springer, 2004.
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Category:Algebraic structures
Category:Commutative algebra
Category:Algebraic number theory

What is the exact set of integers that we are forced to identify with 0?The answer is, unsurprisingly, the set of all integers congruent to 0 modulo ''n''. That is, we must wrap ℤ around itself infinitely many times so that the integers ..., , , , , ... will all align with 0. If we look at what properties this set must satisfy in order to ensure that ℤ