Ring (mathematics)

In mathematics, rings are algebraic structures that generalize fields: multiplication need not be commutative and multiplicative inverses need not exist. In other words, a "ring" is a set equipped with two binary operations satisfying properties analogous to those of addition and multiplication of integers. Ring elements may be numbers such as integers or complex numbers, but they may also be non-numerical objects such as polynomials, square matrices, functions, and power series.
Formally, a "ring" is an abelian group whose operation is called "addition", with a second binary operation called "multiplication" that is associative, is distributive over the addition operation, and has a multiplicative identity element. (Some authors use the term "ring" to refer to the more general structure that omits this last requirement; see .)
Whether a ring is commutative (that is, whether the order in which two elements are multiplied might change the result) has profound implications on its behavior. Commutative algebra, the theory of commutative rings, is a major branch of ring theory. Its development has been greatly influenced by problems and ideas of algebraic number theory and algebraic geometry. The simplest commutative rings are those that admit division by non-zero elements; such rings are called fields. 
Examples of commutative rings include the set of integers with their standard addition and multiplication, the set of polynomials 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 real square matrices with , group rings in representation theory, operator algebras in functional analysis, rings of differential operators, and cohomology rings in topology.
The conceptualization of rings spanned the 1870s to the 1920s, with key contributions by 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. They later proved useful in other branches of mathematics such as geometry and analysis.
Definition.
A ring is a set "R" equipped with two binary operations + (addition) and ⋅ (multiplication) satisfying the following three sets of axioms, called the ring axioms
Notes on the definition.
In the terminology of this article, a ring is defined to have a multiplicative identity, and a structure with the same axiomatic definition but for the requirement of a multiplicative identity is called a rng (IPA: ). For example, the set of even integers with the usual + and ⋅ is a rng, but not a ring. As explained in "" below, many authors apply the term "ring" without requiring a multiplicative identity.
The multiplication symbol ⋅ is usually omitted; for example, "xy" means .
Although ring addition is commutative, ring multiplication is not required to be commutative: "ab" need not necessarily equal "ba". Rings that also satisfy commutativity for multiplication (such as the ring of integers) are called "commutative rings". Books on commutative algebra or algebraic geometry often adopt the convention that "ring" means "commutative ring", to simplify terminology.
In a ring, multiplicative inverses are not required to exist. A nonzero commutative ring in which every nonzero element has a multiplicative inverse is called a field.
The additive group of a ring is the underlying set equipped with only the operation of addition. Although the definition requires that the additive group be abelian, this can be inferred from the other ring axioms. The proof makes use of the "1", and does not work in a rng. (For a rng, omitting the axiom of commutativity of addition leaves it inferrable from the remaining rng assumptions only for elements that are products: .)
Although most modern authors use the term "ring" as defined here, there are a few who use the term to refer to more general structures in which there is no requirement for multiplication to be associative. For these others, every algebra is a "ring".
Illustration.
The most familiar example of a ring is the set of all integers formula_1, consisting of the numbers
The familiar properties for addition and multiplication of integers serve as a model for the axioms of a ring.
Some properties.
Some basic properties of a ring follow immediately from the axioms:
Example: Integers modulo 4.
Equip the set formula_2 with the following operations:
Then Z/4Z 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 Z/4Z, and this element is often denoted by or formula_9, which is consistent with the notation for 0, 1, 2, 3. The additive inverse of any formula_9 in Z/4Z is formula_11. For example, formula_12
Example: 2-by-2 matrices.
The set of 2-by-2 square matrices with entries in a field is
With the operations of matrix addition and matrix multiplication, formula_14 satisfies the above ring axioms. The element formula_15 is the multiplicative identity of the ring. If formula_16 and formula_17, then formula_18 while formula_19; this example shows that the ring is noncommutative.
More generally, for any ring , commutative or not, and any nonnegative integer , the square matrices of dimension with entries in form a ring: see Matrix ring.
History.
Dedekind.
The study of rings originated from the theory of polynomial rings and the theory of algebraic integers. In 1871, Richard Dedekind defined the concept of the ring of integers of a number field. In this context, he introduced the terms "ideal" (inspired by Ernst Kummer's notion of ideal number) and "module" and studied their properties. 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. In 19th century German, the word "Ring" could mean "association", which is still used today in English in a limited sense (for example, spy ring), 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 (in the sense of an equivalence). 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 then , , , , , and so on; in general, "a""n" is going to be an integral linear combination of 1, "a", and "a"2.
Fraenkel and Noether.
The first axiomatic definition of a ring was given by Adolf Fraenkel in 1915, but his axioms were stricter than those in the modern definition. For instance, he required every non-zero-divisor to have a multiplicative inverse. In 1921, Emmy Noether gave a modern axiomatic definition of commutative rings (with and without 1) and developed the foundations of commutative ring theory in her paper "Idealtheorie in Ringbereichen".
Multiplicative identity and the term "ring".
Fraenkel's axioms for a "ring" included that of a multiplicative identity, whereas Noether's did not.
Most or all books on algebra up to around 1960 followed Noether's convention of not requiring a 1 for a "ring". 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, Atiyah and MacDonald, Bourbaki, Eisenbud, and Lang. There are also books published as late as 2006 that use the term without the requirement for a 1.
Gardner and Wiegandt assert that, when dealing with several objects in the category of rings (as opposed to working with a fixed ring), if one requires all rings to have a 1, then some consequences include the lack of existence of infinite direct sums of rings, and 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." 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".
Authors who follow either convention for the use of the term "ring" may use one of the following terms to refer to objects satisfying the other convention:
Basic concepts.
Products and powers.
For each nonnegative integer , given a sequence formula_25 of elements of , one can define the product formula_26 recursively: let and let for .
As a special case, one can define nonnegative integer powers of an element of a ring: and for . Then for all .
Elements in a ring.
A left zero divisor of a ring formula_27 is an element formula_28 in the ring such that there exists a nonzero element formula_29 of formula_27 such that formula_31. A right zero divisor is defined similarly.
A nilpotent element is an element formula_28 such that formula_33 for some formula_34. One example of a nilpotent element is a nilpotent matrix. A nilpotent element in a nonzero ring is necessarily a zero divisor.
An idempotent formula_35 is an element such that formula_36. One example of an idempotent element is a projection in linear algebra.
A unit is an element formula_28 having a multiplicative inverse; in this case the inverse is unique, and is denoted by formula_38. The set of units of a ring is a group under ring multiplication; this group is denoted by formula_39 or formula_40 or formula_41. For example, if "R" is the ring of all square matrices of size "n" over a field, then formula_39 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 called a subring if any one of the following equivalent conditions holds:
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; one could call 2Z a subrng, however.
An intersection of subrings is a subring. Given a subset "E" of "R", the smallest subring of "R" containing "E" is the intersection of all subrings of "R" containing "E", and it is called "the subring generated by E".
For a ring "R", the smallest subring of "R" is called the "characteristic subring" of "R". It can be generated through addition of copies of 1 and −1. It is possible that formula_43 ("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, formula_44 is never zero for any positive integer "n", and those rings are said to have "characteristic zero".
Given a ring "R", let formula_45 denote the set of all elements "x" in "R" such that "x" commutes with every element in "R": formula_46 for any "y" in "R". Then formula_45 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 (each individually) generate a subring of the center.
Ideal.
Let "R" be a ring. A left ideal of "R" is a nonempty subset "I" of "R" such that for any "x", "y" in "I" and "r" in "R", the elements formula_48 and formula_49 are in "I". If formula_50 denotes the "R"-span of "I", that is, the set of finite sums
then "I" is a left ideal if formula_52. Similarly, a right ideal is a subset "I" such that formula_53. 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 formula_54 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 formula_55 and formula_56 are left ideals and right ideals, respectively; they are called the principal left ideals and right ideals generated by "x". The principal ideal formula_57 is written as formula_58. 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 Hopkins–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 formula_59 we have that formula_60 implies either formula_61 or formula_62. Equivalently, "P" is prime if for any ideals formula_63 we have that formula_64 implies either formula_65 or formula_66 This latter formulation illustrates the idea of ideals as generalizations of elements.
Homomorphism.
A homomorphism from a ring to a ring 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:
If one is working with rngs, then the third condition is dropped.
A ring homomorphism "f" is said to be an isomorphism if there exists an inverse homomorphism to "f" (that is, a ring homomorphism that is an inverse function). Any bijective ring homomorphism is a ring isomorphism. Two rings formula_67 are said to be isomorphic if there is an isomorphism between them and in that case one writes formula_68. A ring homomorphism between the same ring is called an endomorphism and an isomorphism between the same ring an automorphism.
Examples:
Given a ring homomorphism formula_73, 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" (which in particular gives a structure of an "A"-module).
Quotient ring.
The notion of quotient ring is analogous to the notion of a quotient group. Given a ring and a two-sided ideal "I" of , view "I" as subgroup of ; then the quotient ring "R"/"I" is the set of cosets of "I" together with the operations
for all "a", "b" in "R". The ring "R"/"I" is also called a factor ring.
As with a quotient group, there is a canonical homomorphism formula_74, given by formula_75. It is surjective and satisfies the following universal property: 
For any ring homomorphism formula_76, invoking the universal property with formula_81 produces a homomorphism formula_82 that gives an isomorphism from formula_83 to the image of .
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 with 1, an -module is an abelian group equipped with an operation (associating an element of to every pair of an element of and an element of ) that satisfies certain axioms. This operation is commonly denoted multiplicatively and called multiplication. The axioms of modules are the following: for all in and all in , we have:
When the ring is noncommutative these axioms define "left modules"; "right modules" are defined similarly by writing instead of . This is not only a change of notation, as the last axiom of right modules (that is ) becomes , 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 , 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 is a ring homomorphism, then is a left module over by the multiplication: . If is commutative or if is contained in the center of , the ring is called a -algebra. In particular, every ring is an algebra over the integers.
Constructions.
Direct product.
Let "R" and "S" be rings. Then the product can be equipped with the following natural ring structure:
for all "r"1, "r"2 in "R" and "s"1, "s"2 in "S". The ring with the above operations of addition and multiplication and the multiplicative identity formula_88 is called the direct product of "R" with "S". The same construction also works for an arbitrary family of rings: if formula_89 are rings indexed by a set "I", then formula_90 is a ring with componentwise addition and multiplication.
Let "R" be a commutative ring and formula_91 be ideals such that formula_92 whenever formula_93. Then the Chinese remainder theorem says there is a canonical ring isomorphism:
A "finite" direct product may also be viewed as a direct sum of ideals. Namely, let formula_95 be rings, formula_96 the inclusions with the images formula_97 (in particular formula_97 are rings though not subrings). Then formula_97 are ideals of "R" and
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 that "R" has the above decomposition. Then we can write
By the conditions on formula_97, one has that formula_103 are central idempotents and formula_104 (orthogonal). Again, one can reverse the construction. Namely, if one is given a partition of 1 in orthogonal central idempotents, then let formula_105, which are two-sided ideals. If each formula_103 is not a sum of orthogonal central idempotents, 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
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 formula_108 of all polynomials in variables formula_109 forms a commutative ring, containing formula_110 as subrings.
If "R" is an integral domain, then formula_111 is also an integral domain; its field of fractions is the field of rational functions. If "R" is a Noetherian ring, then formula_111 is a Noetherian ring. If "R" is a unique factorization domain, then formula_111 is a unique factorization domain. Finally, "R" is a field if and only if formula_111 is a principal ideal domain.
Let formula_115 be commutative rings. Given an element "x" of "S", one can consider the ring homomorphism
(that is, the substitution). If and , then . Because of this, the polynomial "f" is often also denoted by formula_117. The image of the map formula_118 is denoted by formula_119; it is the same thing as the subring of "S" generated by "R" and "x".
Example: formula_120 denotes the image of the homomorphism
In other words, it is the subalgebra of formula_122 generated by "t"2 and "t"3.
Example: let "f" be a polynomial in one variable, that is, an element in a polynomial ring "R". Then formula_123 is an element in formula_124 and formula_125 is divisible by "h" in that ring. The result of substituting zero to "h" in formula_126 is formula_127, 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 formula_128 and an element "x" in "S" there exists a unique ring homomorphism formula_129 such that formula_130 and formula_131 restricts to formula_132. 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 formula_133. The universal property says that this map extends uniquely to
("t" maps to "x") where formula_135 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 formula_111, there exists a ring "S" containing "R" such that "f" is a product of linear factors in formula_137.
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 formula_138 and the set of closed subvarieties of formula_139. 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. Gröbner basis.)
There are some other related constructions. A formal power series ring formula_140 consists of formal power series
together with multiplication and addition that mimic those for convergent series. It contains formula_111 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 M"n"("R"). Given a right "R"-module formula_143, 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 formula_144.
As in linear algebra, a matrix ring may be canonically interpreted as an endomorphism ring: formula_145. This is a special case of the following fact: If formula_146 is an "R"-linear map, then "f" may be written as a matrix with entries formula_147 in formula_148, resulting in the ring isomorphism:
Any ring homomorphism induces .
Schur's lemma says that if "U" is a simple right "R"-module, then formula_144 is a division ring. If formula_151 is a direct sum of "m""i"-copies of simple "R"-modules formula_152, then
The Artin–Wedderburn theorem states any semisimple ring (cf. below) is of this form.
A ring "R" and the matrix ring M"n"("R") over it are Morita equivalent: the category of right modules of "R" is equivalent to the category of right modules over M"n"("R"). In particular, two-sided ideals in "R" correspond in one-to-one to two-sided ideals in M"n"("R").
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 formula_154 defined as follows: it is the disjoint union of all "R""i"'s modulo the equivalence relation formula_155 if and only if formula_156 in "R""i" 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 formula_89, "i" running over positive integers, say, and ring homomorphisms formula_164 such that formula_165 are all the identities and formula_166 is formula_167 whenever formula_168. Then formula_169 is the subring of formula_170 consisting of formula_171 such that formula_172 maps to formula_173 under formula_164.
For an example of a projective limit, see .
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 formula_175 together with the ring homomorphism formula_176 that "inverts" "S"; that is, the homomorphism maps elements in "S" to unit elements in formula_177, and, moreover, any ring homomorphism from "R" that "inverts" "S" uniquely factors through formula_177. The ring formula_177 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 formula_180 consists of elements of the form formula_181 (to be precise, formula_182)
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 formula_183, one often writes formula_184 for formula_177. formula_184 is then a local ring with the maximal ideal formula_187. 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 formula_188 is a prime ideal of a commutative ring "R", then the field of fractions of formula_189 is the same as the residue field of the local ring formula_184 and is denoted by formula_191.
If "M" is a left "R"-module, then the localization of "M" with respect to "S" is given by a change of rings formula_192.
The most important properties of localization are the following: when "R" is a commutative ring and "S" a multiplicatively closed subset
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 formula_177 and "R"-modules map to formula_177-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 formula_204; it is a commutative ring. The canonical homomorphisms from "R" to the quotients formula_205 induce a homomorphism formula_206. 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. The construction is especially useful when "I" is a maximal ideal.
The basic example is the completion of Z at the principal ideal ("p") generated by a prime number "p"; it is called the ring of "p"-adic integers and is denoted Z"p". 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 formula_207 from Q to R given by formula_208 where formula_209 denotes the exponent of "p" in the prime factorization of a nonzero integer "n" into prime numbers (we also put formula_210 and formula_211). It defines a distance function on Q and the completion of Q as a metric space is denoted by Q"p". It is again a field since the field operations extend to the completion. The subring of Q"p" consisting of elements "x" with formula_212 is isomorphic to Z"p".
Similarly, the formal power series ring formula_213 is the completion of formula_111 at formula_215 (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 (that is, free algebra over the integers) with the set "X" of symbols, that is, "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 formula_216 is the unique ring homomorphism. Just as in the group case, every ring can be represented as a quotient of a free ring.
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 formula_217, 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 formula_218 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 formula_219 is an "R"-algebra with multiplication characterized by formula_220. 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 ideal domains, PIDs 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. Let "V" be a finite-dimensional vector space over a field "k" and formula_221 a linear map with minimal polynomial "q". Then, since formula_122 is a unique factorization domain, "q" factors into powers of distinct irreducible polynomials (that is, prime elements):
Letting formula_224, 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 formula_225. Now, if formula_226, then such a cyclic module (for formula_227) has a basis in which the restriction of "f" is represented by a Jordan matrix. Thus, if, say, "k" is algebraically closed, then all formula_227's are of the form formula_229 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.
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; see, for example, the Cartan–Brauer–Hua theorem.
A cyclic algebra, introduced by L. E. Dickson, is a generalization of a quaternion algebra.
Semisimple rings.
A "semisimple module" is a direct sum of simple modules. A "semisimple ring" is a ring that is semisimple as a left module (or right module) over itself. 
Examples.
The Weyl algebra over a field is a simple ring, but it is not semisimple. The same holds for a ring of differential operators in many variables.
Properties.
Any module over a semisimple ring is semisimple. (Proof: A free module over a semisimple ring is semisimple and any module is a quotient of a free module.)
For a ring , the following are equivalent:
Semisimplicity is closely related to separability. A unital associative algebra "A" over a field "k" is said to be separable if the base extension formula_231 is semisimple for every field extension formula_232. 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 formula_233.
The Skolem–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 formula_234. Since formula_235, the similarity is an equivalence relation. The similarity classes formula_236 with the multiplication formula_237 form an abelian group called the Brauer group of "k" and is denoted by formula_238. By the Artin–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, formula_238 is trivial if "k" is a finite field or an algebraically closed field (more generally quasi-algebraically closed field; cf. Tsen's theorem). formula_240 has order 2 (a special case of the theorem of Frobenius). Finally, if "k" is a nonarchimedean local field (for example, formula_241), then formula_242 through the invariant map.
Now, if "F" is a field extension of "k", then the base extension formula_243 induces formula_244. Its kernel is denoted by formula_245. It consists of formula_236 such that formula_231 is a matrix ring over "F" (that is, "A" is split by "F".) If the extension is finite and Galois, then formula_245 is canonically isomorphic to formula_249.
Azumaya 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, The valuation ring of "v" is the subring of "K" consisting of zero and all nonzero "f" such that .
Examples: 
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:
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
a graded ring. There are also homology groups formula_255 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 ()-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 Stanley–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 Stanley–Reisner ring was used to characterize the numbers of faces in each dimension of simplicial polytopes.
Category-theoretic 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 formula_256-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 – where rather than a vector space over a field, one has a "vector space over a ring".
Let 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 and :
where + as in 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, , is an abelian group. Furthermore, for every "r" in "R", right (or left) multiplication by "r" gives rise to a morphism of , by right (or left) distributivity. Let . Consider those endomorphisms of "A", that "factor through" right (or left) multiplication of "R". In other words, let End"R"("A") be the set of all morphisms "m" of "A", having the property that . 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 End"R"("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). 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.
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.
Semiring.
A semiring (sometimes "rig") 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 for all "a" in "R" (since it no longer follows from the other axioms).
Examples:
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 formula_258 (addition), formula_259 (multiplication), formula_260 (additive identity), formula_261 (additive inverse), and formula_262 (multiplicative identity) satisfying the usual ring axioms. Equivalently, a ring object is an object "R" equipped with a factorization of its functor of points formula_263 through the category of rings: formula_264.
Ring scheme.
In algebraic geometry, a ring scheme over a base scheme is a ring object in the category of -schemes. One example is the ring scheme over , which for any commutative ring returns the ring of -isotypic Witt vectors of length over .
Ring spectrum.
In algebraic topology, a ring spectrum is a spectrum "X" together with a multiplication formula_265 and a unit map formula_266 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.
See also.
Special types of rings: