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{{short description|Algebraic structure with two binary operations}}{{about|an algebraic structure|geometric rings|Annulus (mathematics)|the set theory concept|Ring of sets}}File:Chapitel IX. of Die Theorie der algebraischen ZahlkÃ¶rper.png|thumb|Chapter IX of David HilbertDavid HilbertIn mathematics, a ring is one of the fundamental algebraic structures used in abstract algebra. It consists of a set equipped with two binary operations that generalize the arithmetic operations of addition and multiplication. Through this generalization, theorems from arithmetic are extended to non-numerical objects such as polynomials, series, matrices and functions.A ring is an abelian group with a second binary operation that is associative, is distributive over the abelian group operation, and has an identity element (this last property is not required by some authors, see {{slink||Notes on the definition}}). By extension from the integers, the abelian group operation is called addition and the second binary operation is called multiplication.Whether a ring is commutative or not (i.e., whether the order in which two elements are multiplied changes the result or not) has profound implications on its behavior as an abstract object. As a result, commutative ring theory, commonly known as commutative algebra, is a key topic in ring theory. Its development has been greatly influenced by problems and ideas occurring naturally in algebraic number theory and algebraic geometry. Examples of commutative rings include the set of integers equipped with the addition and multiplication operations, the set of polynomials equipped with their addition and multiplication, the coordinate ring of an affine algebraic variety, and the ring of integers of a number field. Examples of noncommutative rings include the ring of n Ã— n real square matrices with n â‰¥ 2, group rings in representation theory, operator algebras in functional analysis, rings of differential operators in the theory of differential operators, and the cohomology ring of a topological space in topology.The conceptualization of rings began in the 1870s and was completed in the 1920s. Key contributors include Dedekind, Hilbert, Fraenkel, and Noether. Rings were first formalized as a generalization of Dedekind domains that occur in number theory, and of polynomial rings and rings of invariants that occur in algebraic geometry and invariant theory. Afterward, they also proved to be useful in other branches of mathematics such as geometry and mathematical analysis.{{Algebraic structures |Ring}}

Definition and illustration

The most familiar example of a ring is the set of all integers, mathbb{Z}, consisting of the numbers
â€¦ , âˆ’5, âˆ’4, âˆ’3, âˆ’2, âˆ’1, 0, 1, 2, 3, 4, 5, â€¦
The familiar properties for addition and multiplication of integers serve as a model for the axioms for rings.

Definition

A ring is a set R equipped with two binary operationsThis means that each operation is defined and produces a unique result in R for each ordered pair of elements of R. + and Â· satisfying the following three sets of axioms, called the ring axiomsBOOK, Nicolas Bourbaki, Algebra, Springer-Verlag, Â§I.8, 1970, BOOK, Algebra, Saunders MacLane, Garrett Birkhoff, AMS Chelsea, 85, 1967, Saunders MacLane, BOOK, Serge Lang, Algebra, Springer-Verlag, 83, 2002, Third, Serge Lang,
1. R is an abelian group under addition, meaning that:
• (a + b) + c = a + (b + c) for all a, b, c in R   (that is, + is associative).
• a + b = b + a for all a, b in R   (that is, + is commutative).
• There is an element 0 in R such that a + 0 = a for all a in R   (that is, 0 is the additive identity).
• For each a in R there exists âˆ’a in R such that a + (âˆ’a) = 0   (that is, âˆ’a is the additive inverse of a).
2. R is a monoid under multiplication, meaning that:
• (a Â· b) Â· c = a Â· (b Â· c) for all a, b, c in R   (that is, Â· is associative).
• There is an element 1 in R such that a Â· 1 = a and 1 Â· a = a for all a in R   (that is, 1 is the multiplicative identity).The existence of 1 is not assumed by some authors; here, the term rng is used if existence of a multiplicative identity is not assumed. See next subsection
3. Multiplication is distributive with respect to addition, meaning that:
• a â‹… (b + c) = (a Â· b) + (a Â· c) for all a, b, c in R   (left distributivity).
• (b + c) Â· a = (b Â· a) + (c Â· a) for all a, b, c in R   (right distributivity).

Basic properties

Some basic properties of a ring follow immediately from the axioms:
• The additive identity, the additive inverse of each element, and the multiplicative identity are unique.
• For any element x in a ring R, one has x0 = 0 = 0x (zero is an absorbing element with respect to multiplication) and (â€“1)x = â€“x.
• If 0 = 1 in a ring R (or more generally, 0 is a unit element), then R has only one element, and is called the zero ring.
• The binomial formula holds for any commuting pair of elements (i.e., any x and y such that xy = yx).

Example: Integers modulo 4

{{see also| Modular arithmetic}}Equip the set mathbf{Z}_4 = left{overline{0}, overline{1}, overline{2}, overline{3}right} with the following operations:
• The sum overline{x} + overline{y} in Z4 is the remainder when the integer x + y is divided by 4 (as x + y is always smaller than 8, this remainder is either x + y or x + y â€“ 4). For example, overline{2} + overline{3} = overline{1} and overline{3} + overline{3} = overline{2}.
• The product overline{x} cdot overline{y} in Z4 is the remainder when the integer xy is divided by 4. For example, overline{2} cdot overline{3} = overline{2} and overline{3} cdot overline{3} = overline{1}.
Then Z4 is a ring: each axiom follows from the corresponding axiom for Z. If x is an integer, the remainder of x when divided by 4 may be considered as an element of Z4, and this element is often denoted by {{nowrap|"x mod 4"}} or overline{x}, which is consistent with the notation for 0, 1, 2, 3. The additive inverse of any overline{x} in Z4 is overline{-x}. For example, -overline{3} = overline{-3} = overline{1}.

Example: 2-by-2 matrices

The set of 2-by-2 matrices with real number entries is written
mathcal{M}_2(mathbb{R}) = left{ left.begin{pmatrix} a & b c & d end{pmatrix} right| a, b, c, d in mathbb{R} right}.
With the operations of matrix addition and matrix multiplication, this set satisfies the above ring axioms. The element begin{pmatrix} 1 & 0 0 & 1 end{pmatrix} is the multiplicative identity of the ring. If A = begin{pmatrix} 0 & 1 1 & 0 end{pmatrix} and B = begin{pmatrix} 0 & 1 0 & 0 end{pmatrix}, then AB = begin{pmatrix} 0 & 0 0 & 1 end{pmatrix} while BA = begin{pmatrix} 1 & 0 0 & 0 end{pmatrix}; this example shows that the ring is noncommutative.More generally, for any ring R, commutative or not, and any nonnegative integer n, one may form the ring of n-by-n matrices with entries in R: see Matrix ring.

History

{{See also|Ring theory#History}}File:Dedekind.jpeg|thumb|100px|right|Richard Dedekind, one of the founders of ring theoryring theory

Dedekind

The study of rings originated from the theory of polynomial rings and the theory of algebraic integers.The development of Ring Theory In 1871, Richard Dedekind defined the concept of the ring of integers of a number field.Kleiner 1998, p. 27. In this context, he introduced the terms "ideal" (inspired by Ernst Kummer's notion of ideal number) and "module" and studied their properties. But Dedekind did not use the term "ring" and did not define the concept of a ring in a general setting.

Hilbert

The term "Zahlring" (number ring) was coined by David Hilbert in 1892 and published in 1897.Hilbert 1897. In 19th century German, the word "Ring" could mean "association", which is still used today in English in a limited sense (e.g., spy ring),Why is a ring called a "ring"? - MathOverflow so if that were the etymology then it would be similar to the way "group" entered mathematics by being a non-technical word for "collection of related things". According to Harvey Cohn, Hilbert used the term for a ring that had the property of "circling directly back" to an element of itself.{{Citation|last=Cohn|first=Harvey|title=Advanced Number Theory|publisher=Dover Publications|location=New York|year=1980|page=49|isbn=978-0-486-64023-5|url=https://archive.org/details/advancednumberth00cohn_0/page/49}} Specifically, in a ring of algebraic integers, all high powers of an algebraic integer can be written as an integral combination of a fixed set of lower powers, and thus the powers "cycle back". For instance, if {{nowrap|1=a3 âˆ’ 4a + 1 = 0}} then {{nowrap|1=a3 = 4a âˆ’ 1}}, {{nowrap|1=a4 = 4a2 âˆ’ a}}, {{nowrap|1=a5 = âˆ’a2 + 16a âˆ’ 4}}, {{nowrap|1=a6 = 16a2 âˆ’ 8a + 1}}, {{nowrap|1=a7 = âˆ’8a2 + 65a âˆ’ 16}}, and so on; in general, an is going to be an integral linear combination of 1, a, and a2.

Fraenkel and Noether

The first axiomatic definition of a ring was given by Adolf Fraenkel in 1914,Fraenkel, pp. 143â€“145Jacobson (2009), p. 86, footnote 1. but his axioms were stricter than those in the modern definition. For instance, he required every non-zero-divisor to have a multiplicative inverse.Fraenkel, p. 144, axiom R8). In 1921, Emmy Noether gave the modern axiomatic definition of (commutative) ring and developed the foundations of commutative ring theory in her paper Idealtheorie in Ringbereichen.Noether, p. 29.

Multiplicative identity: mandatory vs. optional

Fraenkel required a ring to have a multiplicative identity 1,Fraenkel, p. 144, axiom R7). whereas Noether did not.Most or all books on algebraVan der Waerden, 1930.Zariski and Samuel, 1958. up to around 1960 followed Noether's convention of not requiring a 1. Starting in the 1960s, it became increasingly common to see books including the existence of 1 in the definition of ring, especially in advanced books by notable authors such as Artin,Artin, p. 346. Atiyah and MacDonald,Atiyah and MacDonald, p. 1. Bourbaki,Bourbaki, p. 96. Eisenbud,Eisenbud, p. 11. and Lang.Lang, p. 83. But even today, there are many books that do not require a 1.Gallian, p. 235.Hungerford, p. 42.Warner, p. 188.Faced with this terminological ambiguity, some authors have tried to impose their views, while others have tried to adopt more precise terms.In the first category, we find for instance Gardner and Wiegandt, who argue that if one requires all rings to have a 1, then some consequences include the lack of existence of infinite direct sums of rings, and the fact that proper direct summands of rings are not subrings. They conclude that "in many, maybe most, branches of ring theory the requirement of the existence of a unity element is not sensible, and therefore unacceptable."Gardner and Wiegandt 2003. Poonen makes the counterargument that rings without a multiplicative identity are not totally associative (the product of any finite sequence of ring elements, including the empty sequence, is well-defined, independent of the order of operations) and writes "the natural extension of associativity demands that rings should contain an empty product, so it is natural to require rings to have a 1" Poonen 2018..In the second category, we find authors who use the following terms:Wilder 1965, p. 176.Rotman 1998, p. 7.
* rings with multiplicative identity: unital ring, unitary ring, unit ring, ring with unity, ring with identity, or ring with 1 * rings not requiring multiplicative identity: rng or pseudo-ring,Bourbaki, p. 98. although the latter may be confusing, as having other meanings.

Basic examples

Commutative rings

• The prototype example is the ring of integers with the two operations of addition and multiplication.
• The rational, real and complex numbers are commutative rings of a type called fields.
• An algebra over a ring is itself a ring. These are also modules. Some examples:
• The set of all algebraic integers forms a ring. This follows for example from the fact that it is the integral closure of the ring of rational integers in the field of complex numbers. The rings in the three previous examples are subrings of this ring.
• The set of formal power series RX1, â€¦, Xn over a commutative ring R is a ring.
• If S is a set, then the power set of S becomes a ring if we define addition to be the symmetric difference of sets and multiplication to be intersection. This corresponds to a ring of sets and is an example of a Boolean ring.
• The set of all continuous real-valued functions defined on the real line forms a commutative ring. The operations are pointwise addition and multiplication of functions.
• Let X be a set and R a ring. Then the set of all functions from X to R forms a ring, which is commutative if R is commutative. The ring of continuous functions in the previous example is a subring of this ring if X is the real line and R is the field of real numbers.

Noncommutative rings

• For any ring R and any natural number n, the set of all square n-by-n matrices with entries from R, forms a ring with matrix addition and matrix multiplication as operations. For n = 1, this matrix ring is isomorphic to R itself. For n > 1 (and R not the zero ring), this matrix ring is noncommutative.
• If G is an abelian group, then the endomorphisms of G form a ring, the endomorphism ring End(G) of G. The operations in this ring are addition and composition of endomorphisms. More generally, if V is a left module over a ring R, then the set of all R-linear maps forms a ring, also called the endomorphism ring and denoted by EndR(V).
• If G is a group and R is a ring, the group ring of G over R is a free module over R having G as basis. Multiplication is defined by the rules that the elements of G commute with the elements of R and multiply together as they do in the group G.
• Many rings that appear in analysis are noncommutative. For example, most Banach algebras are noncommutative.

Non-rings

{{unordered list
natural numbers N with the usual operations is not a ring, since (N, +) is not even a group (mathematics)>group (the elements are not all invertible with respect to addition). For instance, there is no natural number which can be added to 3 to get 0 as a result. There is a natural way to make it a ring by adding negative numbers to the set, thus obtaining the ring of integers. The natural numbers (including 0) form an algebraic structure known as a semiring (which has all of the properties of a ring except the additive inverse property).| Let R be the set of all continuous functions on the real line that vanish outside a bounded interval depending on the function, with addition as usual but with multiplication defined as convolution:
(f * g)(x) = int_{-infty}^infty f(y)g(x - y)dy.
Then R is a rng, but not a ring: the Dirac delta function has the property of a multiplicative identity, but it is not a function and hence is not an element of R.}}

Basic concepts

Elements in a ring

A left zero divisor of a ring R is a nonzero element a in the ring such that there exists a nonzero element b of R such that ab = 0.This is the definition of Bourbaki. Some other authors such as Lang require a zero divisor to be nonzero. A right zero divisor is defined similarly.A nilpotent element is an element a such that a^n = 0 for some n > 0. One example of a nilpotent element is a nilpotent matrix. A nilpotent element in a nonzero ring is necessarily a zero divisor.An idempotent e is an element such that e^2 = e. One example of an idempotent element is a projection in linear algebra.A unit is an element a having a multiplicative inverse; in this case the inverse is unique, and is denoted by a^{-1}. The set of units of a ring is a group under ring multiplication; this group is denoted by R^times or R^* or U(R). For example, if R is the ring of all square matrices of size n over a field, then R^times consists of the set of all invertible matrices of size n, and is called the general linear group.

Subring

A subset S of R is said to be a subring if it can be regarded as a ring with the addition and the multiplication restricted from R to S. Equivalently, S is a subring if it is not empty, and for any x, y in S, xy, x+y and -x are in S. If all rings have been assumed, by convention, to have a multiplicative identity, then to be a subring one would also require S to share the same identity element as R.In the unital case, like addition and multiplication, the multiplicative identity must be restricted from the original ring. The definition is also equivalent to requiring the set-theoretic inclusion is a ring homomorphism. So if all rings have been assumed to have a multiplicative identity, then a proper ideal is not a subring.For example, the ring Z of integers is a subring of the field of real numbers and also a subring of the ring of polynomials Z[X] (in both cases, Z contains 1, which is the multiplicative identity of the larger rings). On the other hand, the subset of even integers 2Z does not contain the identity element 1 and thus does not qualify as a subring of Z.An intersection of subrings is a subring. The smallest subring containing a given subset E of R is called a subring generated by E. Such a subring exists since it is the intersection of all subrings containing E.For a ring R, the smallest subring containing 1 is called the characteristic subring of R. It can be obtained by adding copies of 1 and âˆ’1 together many times in any mixture. It is possible that ncdot 1=1+1+ldots+1 (n times) can be zero. If n is the smallest positive integer such that this occurs, then n is called the characteristic of R. In some rings, ncdot 1 is never zero for any positive integer n, and those rings are said to have characteristic zero.Given a ring R, let operatorname{Z}(R) denote the set of all elements x in R such that x commutes with every element in R: xy = yx for any y in R. Then operatorname{Z}(R) is a subring of R; called the center of R. More generally, given a subset X of R, let S be the set of all elements in R that commute with every element in X. Then S is a subring of R, called the centralizer (or commutant) of X. The center is the centralizer of the entire ring R. Elements or subsets of the center are said to be central in R; they generate a subring of the center.

Ideal

The definition of an ideal in a ring is analogous to that of normal subgroup in a group. But, in actuality, it plays a role of an idealized generalization of an element in a ring; hence, the name "ideal". Like elements of rings, the study of ideals is central to structural understanding of a ring.Let R be a ring. A nonempty subset I of R is then said to be a left ideal in R if, for any x, y in I and r in R, x+y and rx are in I. If R I denotes the span of I over R; i.e., the set of finite sums
r_1 x_1 + cdots + r_n x_n, quad r_i in R, quad x_i in I,
then I is a left ideal if R I subseteq I. Similarly, I is said to be right ideal if I R subseteq I. A subset I is said to be a two-sided ideal or simply ideal if it is both a left ideal and right ideal. A one-sided or two-sided ideal is then an additive subgroup of R. If E is a subset of R, then R E is a left ideal, called the left ideal generated by E; it is the smallest left ideal containing E. Similarly, one can consider the right ideal or the two-sided ideal generated by a subset of R.If x is in R, then Rx and xR are left ideals and right ideals, respectively; they are called the principal left ideals and right ideals generated by x. The principal ideal RxR is written as (x). For example, the set of all positive and negative multiples of 2 along with 0 form an ideal of the integers, and this ideal is generated by the integer 2. In fact, every ideal of the ring of integers is principal.Like a group, a ring is said to be simple if it is nonzero and it has no proper nonzero two-sided ideals. A commutative simple ring is precisely a field.Rings are often studied with special conditions set upon their ideals. For example, a ring in which there is no strictly increasing infinite chain of left ideals is called a left Noetherian ring. A ring in which there is no strictly decreasing infinite chain of left ideals is called a left Artinian ring. It is a somewhat surprising fact that a left Artinian ring is left Noetherian (the Hopkinsâ€“Levitzki theorem). The integers, however, form a Noetherian ring which is not Artinian.For commutative rings, the ideals generalize the classical notion of divisibility and decomposition of an integer into prime numbers in algebra. A proper ideal P of R is called a prime ideal if for any elements x, yin R we have that xy in P implies either x in P or yin P. Equivalently, P is prime if for any ideals I, J we have that IJ subseteq P implies either I subseteq P or J subseteq P. This latter formulation illustrates the idea of ideals as generalizations of elements.

Homomorphism

A homomorphism from a ring (R, +, Â·) to a ring (S, â€¡, *) is a function f from R to S that preserves the ring operations; namely, such that, for all a, b in R the following identities hold:
• f(a + b) = f(a) â€¡ f(b)
• f(a Â· b) = f(a) f(b)
• f(1R) = 1S
If one is working with not necessarily unital rings, then the third condition is dropped.A ring homomorphism is said to be an isomorphism if there exists an inverse homomorphism to f (i.e., a ring homomorphism which is an inverse function). Any bijective ring homomorphism is a ring isomorphism. Two rings R, S are said to be isomorphic if there is an isomorphism between them and in that case one writes R simeq S. A ring homomorphism between the same ring is called an endomorphism and an isomorphism between the same ring an automorphism.Examples:
• The function that maps each integer x to its remainder modulo 4 (a number in {0, 1, 2, 3}) is a homomorphism from the ring Z to the quotient ring Z/4Z ("quotient ring" is defined below).
• If u is a unit element in a ring R, then R to R, x mapsto uxu^{-1} is a ring homomorphism, called an inner automorphism of R.
• Let R be a commutative ring of prime characteristic p. Then x mapsto x^p is a ring endomorphism of R called the Frobenius homomorphism.
• The Galois group of a field extension L/K is the set of all automorphisms of L whose restrictions to K are the identity.
• For any ring R, there are a unique ring homomorphism Z â†’ R and a unique ring homomorphism R â†’ 0.
• An epimorphism (i.e., right-cancelable morphism) of rings need not be surjective. For example, the unique map mathbb{Z} to mathbb{Q} is an epimorphism.
• An algebra homomorphism from a k-algebra to the endomorphism algebra of a vector space over k is called a representation of the algebra.
Given a ring homomorphism f:R to S, the set of all elements mapped to 0 by f is called the kernel of f. The kernel is a two-sided ideal of R. The image of f, on the other hand, is not always an ideal, but it is always a subring of S.To give a ring homomorphism from a commutative ring R to a ring A with image contained in the center of A is the same as to give a structure of an algebra over R to A (in particular gives a structure of A-module).

Quotient ring

The quotient ring of a ring, is analogous to the notion of a quotient group of a group. More formally, given a ring (R, +, Â· ) and a two-sided ideal I of (R, +, Â· ), the quotient ring (or factor ring) R/I is the set of cosets of I (with respect to the additive group of (R, +, Â· ); i.e. cosets with respect to (R, +)) together with the operations:
(a + I) + (b + I) = (a + b) + I and (a + I)(b + I) = (ab) + I.
for every a, b in R.Like the case of a quotient group, there is a canonical map p: R to R/I given by x mapsto x + I. It is surjective and satisfies the universal property: if f:R to S is a ring homomorphism such that f(I) = 0, then there is a unique overline{f}: R/I to Ssuch that f = overline{f} circ p. In particular, taking I to be the kernel, one sees that the quotient ring R / operatorname{ker} f is isomorphic to the image of f; the fact known as the first isomorphism theorem. The last fact implies that actually any surjective ring homomorphism satisfies the universal property since the image of such a map is a quotient ring.

Module

The concept of a module over a ring generalizes the concept of a vector space (over a field) by generalizing from multiplication of vectors with elements of a field (scalar multiplication) to multiplication with elements of a ring. More precisely, given a ring {{math|R}} with 1, an {{math|R}}-module {{math|M}} is an abelian group equipped with an operation {{math|R × M â†’ M}} (associating an element of {{math|M}} to every pair of an element of {{math|R}} and an element of {{math|M}}) that satisfies certain axioms. This operation is commonly denoted multiplicatively and called multiplication. The axioms of modules are the following: for all {{math|a, b}} in {{math|R}} and all {{math|x, y}} in {{math|M}}, we have:
• {{math|M}} is an abelian group under addition.
• a(x+y)=ax+ay
• (a+b)x=ax+bx
• 1x=x
• (ab)x=a(bx)
When the ring is noncommutative these axioms define left modules; right modules are defined similarly by writing {{math|xa}} instead of {{math|ax}}. This is not only a change of notation, as the last axiom of right modules (that is {{math|1=x(ab) = (xa)b}}) becomes {{math|1=(ab)x = b(ax)}}, if left multiplication (by ring elements) is used for a right module.Basic examples of modules are ideals, including the ring itself.Although similarly defined, the theory of modules is much more complicated than that of vector space, mainly, because, unlike vector spaces, modules are not characterized (up to an isomorphism) by a single invariant (the dimension of a vector space). In particular, not all modules have a basis.The axioms of modules imply that {{math|1=(âˆ’1)x = âˆ’x}}, where the first minus denotes the additive inverse in the ring and the second minus the additive inverse in the module. Using this and denoting repeated addition by a multiplication by a positive integer allows identifying abelian groups with modules over the ring of integers.Any ring homomorphism induces a structure of a module: if {{math|f : R â†’ S}} is a ring homomorphism, then {{math|S}} is a left module over {{math|R}} by the multiplication: {{nowrap|1=rs = f(r)s}}. If {{math|R}} is commutative or if {{math|f(R)}} is contained in the center of {{math|S}}, the ring {{math|S}} is called a {{math|R}}-algebra. In particular, every ring is an algebra over the integers.

Constructions

Direct product

Let R and S be rings. Then the product {{nowrap|R Ã— S}} can be equipped with the following natural ring structure:
• (r1, s1) + (r2, s2) = (r1 + r2, s1 + s2)
• (r1, s1) â‹… (r2, s2) = (r1 â‹… r2, s1 â‹… s2)
for every r1, r2 in R and s1, s2 in S. The ring {{nowrap|R Ã— S}} with the above operations of addition and multiplication and the multiplicative identity (1, 1) is called the direct product of R with S. The same construction also works for an arbitrary family of rings: if R_i are rings indexed by a set I, then prod_{i in I} R_i is a ring with componentwise addition and multiplication.Let R be a commutative ring and mathfrak{a}_1, cdots, mathfrak{a}_n be ideals such that mathfrak{a}_i + mathfrak{a}_j = (1) whenever i ne j. Then the Chinese remainder theorem says there is a canonical ring isomorphism:
R /{textstyle bigcap_{i=1}^{n}{mathfrak{a}_i}} simeq prod_{i=1}^{n}{R/ mathfrak{a}_i}, qquad x ;operatorname{mod}; {textstyle bigcap_{i=1}^{n}{mathfrak{a}_i}} mapsto (x ;operatorname{mod}; mathfrak{a}_1, ldots , x ;operatorname{mod}; mathfrak{a}_n).
A "finite" direct product may also be viewed as a direct sum of ideals.{{harvnb|Cohn|2003|loc=Theorem 4.5.1}} Namely, let R_i, 1 le i le n be rings, R_i to R = prod R_i the inclusions with the images mathfrak{a}_i (in particular mathfrak{a}_i are rings though not subrings). Then mathfrak{a}_i are ideals of R and
R = mathfrak{a}_1 oplus cdots oplus mathfrak{a}_n, quad mathfrak{a}_i mathfrak{a}_j = 0, i ne j, quad mathfrak{a}_i^2 subseteq mathfrak{a}_i
as a direct sum of abelian groups (because for abelian groups finite products are the same as direct sums). Clearly the direct sum of such ideals also defines a product of rings that is isomorphic to R. Equivalently, the above can be done through central idempotents. Assume R has the above decomposition. Then we can write
1 = e_1 + cdots + e_n, quad e_i in mathfrak{a}_i.
By the conditions on mathfrak{a}_i, one has that e_i are central idempotents and e_i e_j = 0, i ne j (orthogonal). Again, one can reverse the construction. Namely, if one is given a partition of 1 in orthogonal central idempotents, then let mathfrak{a}_i = R e_i, which are two-sided ideals. If each e_i is not a sum of orthogonal central idempotents,such a central idempotent is called centrally primitive. then their direct sum is isomorphic to R.An important application of an infinite direct product is the construction of a projective limit of rings (see below). Another application is a restricted product of a family of rings (cf. adele ring).

Polynomial ring

Given a symbol t (called a variable) and a commutative ring R, the set of polynomials
R[t] = left{ a_n t^n + a_{n-1} t^{n -1} + dots + a_1 t + a_0 mid n ge 0, a_j in R right}
forms a commutative ring with the usual addition and multiplication, containing R as a subring. It is called the polynomial ring over R. More generally, the set Rleft[t_1, ldots, t_nright] of all polynomials in variables t_1, ldots, t_n forms a commutative ring, containing Rleft[t_iright] as subrings.If R is an integral domain, then R[t] is also an integral domain; its field of fractions is the field of rational functions. If R is a noetherian ring, then R[t] is a noetherian ring. If R is a unique factorization domain, then R[t] is a unique factorization domain. Finally, R is a field if and only if R[t] is a principal ideal domain.Let R subseteq S be commutative rings. Given an element x of S, one can consider the ring homomorphism
R[t] to S, quad f mapsto f(x)
(i.e., the substitution). If S = R[t] and x = t, then f(t) = f. Because of this, the polynomial f is often also denoted by f(t). The image of the map f mapsto f(x) is denoted by R[x]; it is the same thing as the subring of S generated by R and x.Example: kleft[t^2, t^3right] denotes the image of the homomorphism
k[x, y] to k[t], , f mapsto fleft(t^2, t^3right).
In other words, it is the subalgebra of k[t] generated by t2 and t3.Example: let f be a polynomial in one variable; i.e., an element in a polynomial ring R. Then f(x+h) is an element in R[h] and f(x + h) - f(x) is divisible by h in that ring. The result of substituting zero to h in (f(x + h) - f(x))/h is f'(x), the derivative of f at x.The substitution is a special case of the universal property of a polynomial ring. The property states: given a ring homomorphism phi: R to S and an element x in S there exists a unique ring homomorphism overline{phi}: R[t] to S such that overline{phi}(t) = x and overline{phi} restricts to phi.{{harvnb|Jacobson|1974|loc=Theorem 2.10}} For example, choosing a basis, a symmetric algebra satisfies the universal property and so is a polynomial ring.To give an example, let S be the ring of all functions from R to itself; the addition and the multiplication are those of functions. Let x be the identity function. Each r in R defines a constant function, giving rise to the homomorphism R to S. The universal property says that this map extends uniquely to
R[t] to S, quad f mapsto overline{f}
(t maps to x) where overline{f} is the polynomial function defined by f. The resulting map is injective if and only if R is infinite.Given a non-constant monic polynomial f in R[t], there exists a ring S containing R such that f is a product of linear factors in S[t].{{harvnb|Bourbaki|AlgÃ¨bre commutative|loc=Ch 5. Â§1, Lemma 2}}Let k be an algebraically closed field. The Hilbert's Nullstellensatz (theorem of zeros) states that there is a natural one-to-one correspondence between the set of all prime ideals in kleft[t_1, ldots, t_nright] and the set of closed subvarieties of k^n. In particular, many local problems in algebraic geometry may be attacked through the study of the generators of an ideal in a polynomial ring. (cf. GrÃ¶bner basis.)There are some other related constructions. A formal power series ring R[![t]!] consists of formal power series
sum_0^infty a_i t^i, quad a_i in R
together with multiplication and addition that mimic those for convergent series. It contains R[t] as a subring. A formal power series ring does not have the universal property of a polynomial ring; a series may not converge after a substitution. The important advantage of a formal power series ring over a polynomial ring is that it is local (in fact, complete).

Matrix ring and endomorphism ring

Let R be a ring (not necessarily commutative). The set of all square matrices of size n with entries in R forms a ring with the entry-wise addition and the usual matrix multiplication. It is called the matrix ring and is denoted by Mn(R). Given a right R-module U, the set of all R-linear maps from U to itself forms a ring with addition that is of function and multiplication that is of composition of functions; it is called the endomorphism ring of U and is denoted by operatorname{End}_R(U).As in linear algebra, a matrix ring may be canonically interpreted as an endomorphism ring: operatorname{End}_R(R^n) simeq operatorname{M}_n(R). This is a special case of the following fact: If f: oplus_1^n U to oplus_1^n U is an R-linear map, then f may be written as a matrix with entries f_{ij} in S = operatorname{End}_R(U), resulting in the ring isomorphism:
operatorname{End}_R(oplus_1^n U) to operatorname{M}_n(S), quad f mapsto (f_{ij}).
Any ring homomorphism R â†’ S induces {{nowrap|Mn(R) â†’ Mn(S)}}; in fact, any ring homomorphism between matrix rings arises in this way.{{harvnb|Cohn|2003|loc=4.4}}Schur's lemma says that if U is a simple right R-module, then operatorname{End}_R(U) is a division ring.{{harvnb|Lang|2002|loc=Ch. XVII. Proposition 1.1.}} If displaystyle U = bigoplus_{i = 1}^r U_i^{oplus m_i} is a direct sum of mi-copies of simple R-modules U_i, then
operatorname{End}_R(U) simeq bigoplus_1^r operatorname{M}_{m_i} (operatorname{End}_R(U_i)).
The Artinâ€“Wedderburn theorem states any semisimple ring (cf. below) is of this form.A ring R and the matrix ring Mn(R) over it are Morita equivalent: the category of right modules of R is equivalent to the category of right modules over Mn(R). In particular, two-sided ideals in R correspond in one-to-one to two-sided ideals in Mn(R).Examples:

Limits and colimits of rings

Let R'i be a sequence of rings such that R'i is a subring of R'i+1 for all i. Then the union (or filtered colimit) of R'i is the ring varinjlim R_i defined as follows: it is the disjoint union of all R'is modulo the equivalence relation x sim y if and only if x = y in Ri for sufficiently large i''.Examples of colimits:
Any commutative ring is the colimit of finitely generated subrings.A projective limit (or a filtered limit) of rings is defined as follows. Suppose we're given a family of rings R_i, i running over positive integers, say, and ring homomorphisms R_j to R_i, j ge i such that R_i to R_i are all the identities and R_k to R_j to R_i is R_k to R_i whenever k ge j ge i. Then varprojlim R_i is the subring of prod R_i consisting of (x_n) such that x_j maps to x_i under R_j to R_i, j ge i.For an example of a projective limit, see {{slink||Completion}}.

Localization

The localization generalizes the construction of the field of fractions of an integral domain to an arbitrary ring and modules. Given a (not necessarily commutative) ring R and a subset S of R, there exists a ring R[S^{-1}] together with the ring homomorphism R to Rleft[S^{-1}right] that "inverts" S; that is, the homomorphism maps elements in S to unit elements in Rleft[S^{-1}right], and, moreover, any ring homomorphism from R that "inverts" S uniquely factors through Rleft[S^{-1}right].{{harvnb|Cohn|1995|loc=Proposition 1.3.1.}} The ring Rleft[S^{-1}right] is called the localization of R with respect to S. For example, if R is a commutative ring and f an element in R, then the localization Rleft[f^{-1}right] consists of elements of the form r/f^n, , r in R , , n ge 0 (to be precise, Rleft[f^{-1}right] = R[t]/(tf - 1).){{harvnb|Eisenbud|2004|loc=Exercise 2.2}}The localization is frequently applied to a commutative ring R with respect to the complement of a prime ideal (or a union of prime ideals) in R. In that case S = R - mathfrak{p}, one often writes R_mathfrak{p} for Rleft[S^{-1}right]. R_mathfrak{p} is then a local ring with the maximal ideal mathfrak{p} R_mathfrak{p}. This is the reason for the terminology "localization". The field of fractions of an integral domain R is the localization of R at the prime ideal zero. If mathfrak{p} is a prime ideal of a commutative ring R, then the field of fractions of R/mathfrak{p} is the same as the residue field of the local ring R_mathfrak{p} and is denoted by k(mathfrak{p}).If M is a left R-module, then the localization of M with respect to S is given by a change of rings Mleft[S^{-1}right] = Rleft[S^{-1}right] otimes_R M.The most important properties of localization are the following: when R is a commutative ring and S a multiplicatively closed subset
• mathfrak{p} mapsto mathfrak{p}left[S^{-1}right] is a bijection between the set of all prime ideals in R disjoint from S and the set of all prime ideals in Rleft[S^{-1}right].{{harvnb|Milne|2012|loc=Proposition 6.4}}
• Rleft[S^{-1}right] = varinjlim Rleft[f^{-1}right], f running over elements in S with partial ordering given by divisibility.{{harvnb|Milne|2012|loc=The end of Chapter 7}}
• The localization is exact:
• : 0 to M'left[S^{-1}right] to Mleft[S^{-1}right] to Mleft[S^{-1}right] to 0 is exact over Rleft[S^{-1}right] whenever 0 to M' to M to M to 0 is exact over R.
• Conversely, if 0 to M'_mathfrak{m} to M_mathfrak{m} to M_mathfrak{m} to 0 is exact for any maximal ideal mathfrak{m}, then 0 to M' to M to M to 0 is exact.
• A remark: localization is no help in proving a global existence. One instance of this is that if two modules are isomorphic at all prime ideals, it does not follow that they are isomorphic. (One way to explain this is that the localization allows one to view a module as a sheaf over prime ideals and a sheaf is inherently a local notion.)
In category theory, a localization of a category amounts to making some morphisms isomorphisms. An element in a commutative ring R may be thought of as an endomorphism of any R-module. Thus, categorically, a localization of R with respect to a subset S of R is a functor from the category of R-modules to itself that sends elements of S viewed as endomorphisms to automorphisms and is universal with respect to this property. (Of course, R then maps to Rleft[S^{-1}right] and R-modules map to Rleft[S^{-1}right]-modules.)

Completion

Let R be a commutative ring, and let I be an ideal of R.The completion of R at I is the projective limit hat{R} = varprojlim R/I^n; it is a commutative ring. The canonical homomorphisms from R to the quotients R/I^n induce a homomorphism R to hat{R}. The latter homomorphism is injective if R is a noetherian integral domain and I is a proper ideal, or if R is a noetherian local ring with maximal ideal I, by Krull's intersection theorem.Atiyah and Macdonald, Theorem 10.17 and its corollaries. The construction is especially useful when I is a maximal ideal.The basic example is the completion Zp of Z at the principal ideal (p) generated by a prime number p; it is called the ring of p-adic integers. The completion can in this case be constructed also from the p-adic absolute value on Q. The p-adic absolute value on Q is a map x mapsto |x| from Q to R given by |n|_p=p^{-v_p(n)} where v_p(n) denotes the exponent of p in the prime factorization of a nonzero integer n into prime numbers (we also put |0|_p=0 and |m/n|_p = |m|_p/|n|_p). It defines a distance function on Q and the completion of Q as a metric space is denoted by Qp. It is again a field since the field operations extend to the completion. The subring of Qp consisting of elements x with |x|_p le 1 is isomorphic to Zp.Similarly, the formal power series ring R[{[t]}] is the completion of R[t] at (t) (see also Hensel's lemma)A complete ring has much simpler structure than a commutative ring. This owns to the Cohen structure theorem, which says, roughly, that a complete local ring tends to look like a formal power series ring or a quotient of it. On the other hand, the interaction between the integral closure and completion has been among the most important aspects that distinguish modern commutative ring theory from the classical one developed by the likes of Noether. Pathological examples found by Nagata led to the reexamination of the roles of Noetherian rings and motivated, among other things, the definition of excellent ring.

Rings with generators and relations

The most general way to construct a ring is by specifying generators and relations. Let F be a free ring (i.e., free algebra over the integers) with the set X of symbols; i.e., F consists of polynomials with integral coefficients in noncommuting variables that are elements of X. A free ring satisfies the universal property: any function from the set X to a ring R factors through F so that F to R is the unique ring homomorphism. Just as in the group case, every ring can be represented as a quotient of a free ring.{{harvnb|Cohn|1995|loc=pg. 242}}.Now, we can impose relations among symbols in X by taking a quotient. Explicitly, if E is a subset of F, then the quotient ring of F by the ideal generated by E is called the ring with generators X and relations E. If we used a ring, say, A as a base ring instead of Z, then the resulting ring will be over A. For example, if E = { xy - yx mid x, y in X }, then the resulting ring will be the usual polynomial ring with coefficients in A in variables that are elements of X (It is also the same thing as the symmetric algebra over A with symbols X.)In the category-theoretic terms, the formation S mapsto text{the free ring generated by the set } S is the left adjoint functor of the forgetful functor from the category of rings to Set (and it is often called the free ring functor.)Let A, B be algebras over a commutative ring R. Then the tensor product of R-modules A otimes_R B is a R-module. We can turn it to a ring by extending linearly (x otimes u) (y otimes v) = xy otimes uv. See also: tensor product of algebras, change of rings.

Special kinds of rings

Domains

A nonzero ring with no nonzero zero-divisors is called a domain. A commutative domain is called an integral domain. The most important integral domains are principal ideals domains, PID for short, and fields. A principal ideal domain is an integral domain in which every ideal is principal. An important class of integral domains that contain a PID is a unique factorization domain (UFD), an integral domain in which every nonunit element is a product of prime elements (an element is prime if it generates a prime ideal.) The fundamental question in algebraic number theory is on the extent to which the ring of (generalized) integers in a number field, where an "ideal" admits prime factorization, fails to be a PID.Among theorems concerning a PID, the most important one is the structure theorem for finitely generated modules over a principal ideal domain. The theorem may be illustrated by the following application to linear algebra.{{harvnb|Lang|2002|loc=Ch XIV, Â§2}} Let V be a finite-dimensional vector space over a field k and f: V to V a linear map with minimal polynomial q. Then, since k[t] is a unique factorization domain, q factors into powers of distinct irreducible polynomials (i.e., prime elements):
q = p_1^{e_1} ldots p_s^{e_s}.
Letting t cdot v = f(v), we make V a k[t]-module. The structure theorem then says V is a direct sum of cyclic modules, each of which is isomorphic to the module of the form k[t]/left(p_i^{k_j}right). Now, if p_i(t) = t - lambda_i, then such a cyclic module (for p_i) has a basis in which the restriction of f is represented by a Jordan matrix. Thus, if, say, k is algebraically closed, then all p_i's are of the form t - lambda_i and the above decomposition corresponds to the Jordan canonical form of f.In algebraic geometry, UFDs arise because of smoothness. More precisely, a point in a variety (over a perfect field) is smooth if the local ring at the point is a regular local ring. A regular local ring is a UFD.{{harvnb|Weibel|loc=Ch 1, Theorem 3.8}}The following is a chain of class inclusions that describes the relationship between rings, domains and fields:

Division ring

A division ring is a ring such that every non-zero element is a unit. A commutative division ring is a field. A prominent example of a division ring that is not a field is the ring of quaternions. Any centralizer in a division ring is also a division ring. In particular, the center of a division ring is a field. It turned out that every finite domain (in particular finite division ring) is a field; in particular commutative (the Wedderburn's little theorem).Every module over a division ring is a free module (has a basis); consequently, much of linear algebra can be carried out over a division ring instead of a field.The study of conjugacy classes figures prominently in the classical theory of division rings. Cartan famously asked the following question: given a division ring D and a proper sub-division-ring S that is not contained in the center, does each inner automorphism of D restrict to an automorphism of S? The answer is negative: this is the Cartanâ€“Brauerâ€“Hua theorem.A cyclic algebra, introduced by L. E. Dickson, is a generalization of a quaternion algebra.

Semisimple rings

A ring is called a semisimple ring if it is semisimple as a left module (or right module) over itself; i.e., a direct sum of simple modules. A ring is called a semiprimitive ring if its Jacobson radical is zero. (The Jacobson radical is the intersection of all maximal left ideals.) A ring is semisimple if and only if it is artinian and is semiprimitive.An algebra over a field k is artinian if and only if it has finite dimension. Thus, a semisimple algebra over a field is necessarily finite-dimensional, while a simple algebra may have infinite dimension; e.g., the ring of differential operators.Any module over a semisimple ring is semisimple. (Proof: any free module over a semisimple ring is clearly semisimple and any module is a quotient of a free module.)Examples of semisimple rings:
• A matrix ring over a division ring is semisimple (actually simple).
• The group ring k[G] of a finite group G over a field k is semisimple if the characteristic of k does not divide the order of G. (Maschke's theorem)
• The Weyl algebra (over a field) is a simple ring; it is not semisimple since it has infinite dimension and thus not artinian.
• Clifford algebras are semisimple.
Semisimplicity is closely related to separability. An algebra A over a field k is said to be separable if the base extension A otimes_k F is semisimple for any field extension F/k. If A happens to be a field, then this is equivalent to the usual definition in field theory (cf. separable extension.)

Central simple algebra and Brauer group

For a field k, a k-algebra is central if its center is k and is simple if it is a simple ring. Since the center of a simple k-algebra is a field, any simple k-algebra is a central simple algebra over its center. In this section, a central simple algebra is assumed to have finite dimension. Also, we mostly fix the base field; thus, an algebra refers to a k-algebra. The matrix ring of size n over a ring R will be denoted by R_n.The Skolemâ€“Noether theorem states any automorphism of a central simple algebra is inner.Two central simple algebras A and B are said to be similar if there are integers n and m such that A otimes_k k_n approx B otimes_k k_m.{{harvnb|Milne|CFT|loc=Ch IV, Â§2}} Since k_n otimes_k k_m simeq k_{nm}, the similarity is an equivalence relation. The similarity classes [A] with the multiplication [A][B] = left[A otimes_k Bright] form an abelian group called the Brauer group of k and is denoted by operatorname{Br}(k). By the Artinâ€“Wedderburn theorem, a central simple algebra is the matrix ring of a division ring; thus, each similarity class is represented by a unique division ring.For example, operatorname{Br}(k) is trivial if k is a finite field or an algebraically closed field (more generally quasi-algebraically closed field; cf. Tsen's theorem). operatorname{Br}(mathbb{R}) has order 2 (a special case of the theorem of Frobenius). Finally, if k is a nonarchimedean local field (e.g., mathbb{Q}_p), then operatorname{Br}(k) = mathbb{Q}/mathbb{Z} through the invariant map.Now, if F is a field extension of k, then the base extension - otimes_k F induces operatorname{Br}(k) to operatorname{Br}(F). Its kernel is denoted by operatorname{Br}(F/k). It consists of [A] such that A otimes_k F is a matrix ring over F (i.e., A is split by F.) If the extension is finite and Galois, then operatorname{Br}(F/k) is canonically isomorphic to H^2left(operatorname{Gal}(F/k), k^*right).Serre, J-P ., Applications algÃ©briques de la cohomologie des groupes, I, II, SÃ©minaire Henri Cartan, 1950/51 weblinkAzumaya algebras generalize the notion of central simple algebras to a commutative local ring.

Valuation ring

If K is a field, a valuation v is a group homomorphism from the multiplicative group K* to a totally ordered abelian group G such that, for any f, g in K with f + g nonzero, v(f + g) â‰¥ min{v(f), v(g)}. The valuation ring of v is the subring of K consisting of zero and all nonzero f such that v(f) â‰¥ 0.Examples: {{unordered list| The field of formal Laurent series k(!(t)!) over a field k comes with the valuation v such that v(f) is the least degree of a nonzero term in f; the valuation ring of v is the formal power series ring k[![t]!].| More generally, given a field k and a totally ordered abelian group G, let k(!(G)!) be the set of all functions from G to k whose supports (the sets of points at which the functions are nonzero) are well ordered. It is a field with the multiplication given by convolution:
(f*g)(t) = sum_{s in G} f(s)g(t - s).
It also comes with the valuation v such that v(f) is the least element in the support of f. The subring consisting of elements with finite support is called the group ring of G (which makes sense even if G is not commutative). If G is the ring of integers, then we recover the previous example (by identifying f with the series whose n-th coefficient is f(n).)}}See also: Novikov ring and uniserial ring.

Rings with extra structure

A ring may be viewed as an abelian group (by using the addition operation), with extra structure: namely, ring multiplication. In the same way, there are other mathematical objects which may be considered as rings with extra structure. For example:
• An associative algebra is a ring that is also a vector space over a field K such that the scalar multiplication distributes over the ring multiplication. For instance, the set of n-by-n matrices over the real field R has dimension n2 as a real vector space.
• A ring R is a topological ring if its set of elements R is given a topology which makes the addition map ( + : Rtimes R to R,) and the multiplication map ( cdot : Rtimes R to R,) to be both continuous as maps between topological spaces (where X Ã— X inherits the product topology or any other product in the category). For example, n-by-n matrices over the real numbers could be given either the Euclidean topology, or the Zariski topology, and in either case one would obtain a topological ring.
• A Î»-ring is a commutative ring R together with operations Î»n: R â†’ R that are like n-th exterior powers:

lambda^n(x + y) = sum_0^n lambda^i(x) lambda^{n-i}(y).
For example, Z is a Î»-ring with lambda^n(x) = binom{x}{n}, the binomial coefficients. The notion plays a central rule in the algebraic approach to the Riemannâ€“Roch theorem.

Some examples of the ubiquity of rings

Many different kinds of mathematical objects can be fruitfully analyzed in terms of some associated ring.

Cohomology ring of a topological space

To any topological space X one can associate its integral cohomology ring
H^*(X,mathbb{Z}) = bigoplus_{i=0}^{infty} H^i(X,mathbb{Z}),
a graded ring. There are also homology groups H_i(X,mathbb{Z}) of a space, and indeed these were defined first, as a useful tool for distinguishing between certain pairs of topological spaces, like the spheres and tori, for which the methods of point-set topology are not well-suited. Cohomology groups were later defined in terms of homology groups in a way which is roughly analogous to the dual of a vector space. To know each individual integral homology group is essentially the same as knowing each individual integral cohomology group, because of the universal coefficient theorem. However, the advantage of the cohomology groups is that there is a natural product, which is analogous to the observation that one can multiply pointwise a k-multilinear form and an l-multilinear form to get a (k + l)-multilinear form.The ring structure in cohomology provides the foundation for characteristic classes of fiber bundles, intersection theory on manifolds and algebraic varieties, Schubert calculus and much more.

Burnside ring of a group

To any group is associated its Burnside ring which uses a ring to describe the various ways the group can act on a finite set. The Burnside ring's additive group is the free abelian group whose basis are the transitive actions of the group and whose addition is the disjoint union of the action. Expressing an action in terms of the basis is decomposing an action into its transitive constituents. The multiplication is easily expressed in terms of the representation ring: the multiplication in the Burnside ring is formed by writing the tensor product of two permutation modules as a permutation module. The ring structure allows a formal way of subtracting one action from another. Since the Burnside ring is contained as a finite index subring of the representation ring, one can pass easily from one to the other by extending the coefficients from integers to the rational numbers.

Representation ring of a group ring

To any group ring or Hopf algebra is associated its representation ring or "Green ring". The representation ring's additive group is the free abelian group whose basis are the indecomposable modules and whose addition corresponds to the direct sum. Expressing a module in terms of the basis is finding an indecomposable decomposition of the module. The multiplication is the tensor product. When the algebra is semisimple, the representation ring is just the character ring from character theory, which is more or less the Grothendieck group given a ring structure.

Function field of an irreducible algebraic variety

To any irreducible algebraic variety is associated its function field. The points of an algebraic variety correspond to valuation rings contained in the function field and containing the coordinate ring. The study of algebraic geometry makes heavy use of commutative algebra to study geometric concepts in terms of ring-theoretic properties. Birational geometry studies maps between the subrings of the function field.

Face ring of a simplicial complex

Every simplicial complex has an associated face ring, also called its Stanleyâ€“Reisner ring. This ring reflects many of the combinatorial properties of the simplicial complex, so it is of particular interest in algebraic combinatorics. In particular, the algebraic geometry of the Stanleyâ€“Reisner ring was used to characterize the numbers of faces in each dimension of simplicial polytopes.

Category theoretical description

Every ring can be thought of as a monoid in Ab, the category of abelian groups (thought of as a monoidal category under the tensor product of {mathbb Z}-modules). The monoid action of a ring R on an abelian group is simply an R-module. Essentially, an R-module is a generalization of the notion of a vector space â€“ where rather than a vector space over a field, one has a "vector space over a ring".Let (A, +) be an abelian group and let End(A) be its endomorphism ring (see above). Note that, essentially, End(A) is the set of all morphisms of A, where if f is in End(A), and g is in End(A), the following rules may be used to compute f + g and f Â· g:
• (f + g)(x) = f(x) + g(x)
• (f Â· g)(x) = f(g(x))
where + as in f(x) + g(x) is addition in A, and function composition is denoted from right to left. Therefore, associated to any abelian group, is a ring. Conversely, given any ring, (R, +, Â· ), (R, +) is an abelian group. Furthermore, for every r in R, right (or left) multiplication by r gives rise to a morphism of (R, +), by right (or left) distributivity. Let A = (R, +). Consider those endomorphisms of A, that "factor through" right (or left) multiplication of R. In other words, let EndR(A) be the set of all morphisms m of A, having the property that m(r Â· x) = r Â· m(x). It was seen that every r in R gives rise to a morphism of A: right multiplication by r. It is in fact true that this association of any element of R, to a morphism of A, as a function from R to EndR(A), is an isomorphism of rings. In this sense, therefore, any ring can be viewed as the endomorphism ring of some abelian X-group (by X-group, it is meant a group with X being its set of operators).Jacobson (2009), p. 162, Theorem 3.2. In essence, the most general form of a ring, is the endomorphism group of some abelian X-group.Any ring can be seen as a preadditive category with a single object. It is therefore natural to consider arbitrary preadditive categories to be generalizations of rings. And indeed, many definitions and theorems originally given for rings can be translated to this more general context. Additive functors between preadditive categories generalize the concept of ring homomorphism, and ideals in additive categories can be defined as sets of morphisms closed under addition and under composition with arbitrary morphisms.

Generalization

Algebraists have defined structures more general than rings by weakening or dropping some of ring axioms.

Rng

A rng is the same as a ring, except that the existence of a multiplicative identity is not assumed.Jacobson 2009.

Nonassociative ring

A nonassociative ring is an algebraic structure that satisfies all of the ring axioms except the associative property and the existence of a multiplicative identity. A notable example is a Lie algebra. There exists some structure theory for such algebras that generalizes the analogous results for Lie algebras and associative algebras.{{citation needed|date=November 2013}}

Semiring

A semiring is obtained by weakening the assumption that (R,+) is an abelian group to the assumption that (R,+) is a commutative monoid, and adding the axiom that 0 Â· a = a Â· 0 = 0 for all a in R (since it no longer follows from the other axioms).Examples:
• the non-negative integers {0,1,2,ldots} with ordinary addition and multiplication;
• the tropical semiring.

Other ring-like objects

Ring object in a category

Let C be a category with finite products. Let pt denote a terminal object of C (an empty product). A ring object in C is an object R equipped with morphisms R times R stackrel{a}to R (addition), R times R stackrel{m}to R (multiplication), operatorname{pt} stackrel{0}to R (additive identity), R stackrel{i}to R (additive inverse), and operatorname{pt} stackrel{1}to R (multiplicative identity) satisfying the usual ring axioms. Equivalently, a ring object is an object R equipped with a factorization of its functor of points h_R = operatorname{Hom}(-,R) : C^{operatorname{op}} to mathbf{Sets} through the category of rings: C^{operatorname{op}} to mathbf{Rings} stackrel{textrm{forgetful}}longrightarrow mathbf{Sets}.

Ring scheme

In algebraic geometry, a ring scheme over a base scheme {{math|S}} is a ring object in the category of {{math|S}}-schemes. One example is the ring scheme {{math|Wn}} over {{math|Spec Z}}, which for any commutative ring {{math|A}} returns the ring {{math|Wn(A)}} of {{math|p}}-isotypic Witt vectors of length {{math|n}} over {{math|A}}.Serre, p. 44.

Ring spectrum

In algebraic topology, a ring spectrum is a spectrum X together with a multiplication mu colon X wedge X to X and a unit map S to X from the sphere spectrum S, such that the ring axiom diagrams commute up to homotopy. In practice, it is common to define a ring spectrum as a monoid object in a good category of spectra such as the category of symmetric spectra.

{{Div col|colwidth=18em}} {{Div col end}}Special types of rings:{{Div col|colwidth=22em}} {{Div col end}}

Notes

{{cnote|a|Some authors only require that a ring be a semigroup under multiplication; that is, do not require that there be a multiplicative identity (1). See the section Notes on the definition for more details.}}{{cnote|b|Elements which do have multiplicative inverses are called units, see {{Harvard citations|nb = yes|last = Lang|year = 2002|loc =Â§II.1, p. 84}}.}}{{cnote|c|The closure axiom is already implied by the condition that +/â€¢ be a binary operation. Some authors therefore omit this axiom. {{Harvard citations|nb = yes|last = Lang|year = 2002}}}}{{cnote|d|The transition from the integers to the rationals by adding fractions is generalized by the quotient field.}}{{cnote|e|Many authors include commutativity of rings in the set of ring axioms (see above) and therefore refer to "commutative rings" as just "rings".}}

{{reflist|30em}}

References

General references

• BOOK

, Artin
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Historical references

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