# Quotient ring

# Quotient ring

In ring theory, a branch of abstract algebra, a **quotient ring**, also known as **factor ring**, **difference ring**^{[1]} or **residue class ring**, is a construction quite similar to the quotient groups of group theory and the quotient spaces of linear algebra.^{[2]}^{[3]} It is a specific example of a quotient, as viewed from the general setting of universal algebra. One starts with a ring *R* and a two-sided ideal *I* in *R*, and constructs a new ring, the quotient ring *R* / *I*, whose elements are the cosets of *I* in *R* subject to special *+* and *⋅* operations.

Quotient rings are distinct from the so-called 'quotient field', or field of fractions, of an integral domain as well as from the more general 'rings of quotients' obtained by localization.

Formal quotient ring construction

Given a ring *R* and a two-sided ideal *I* in *R*, we may define an equivalence relation ~ on *R* as follows:

*a*~

*b*if and only if

*a*−

*b*is in

*I*.

Using the ideal properties, it is not difficult to check that ~ is a congruence relation.
In case *a* ~ *b*, we say that *a* and *b* are *congruent modulo* *I*.
The equivalence class of the element *a* in *R* is given by

- [

*a*] =

*a*+

*I*:= {

*a*+

*r*:

*r*in

*I*}.

This equivalence class is also sometimes written as *a* mod *I* and called the "residue class of *a* modulo *I*".

The set of all such equivalence classes is denoted by *R* / *I*; it becomes a ring, the **factor ring** or **quotient ring** of *R* modulo *I*, if one defines

(

*a*+*I*) + (*b*+*I*) = (*a*+*b*) +*I*;(

*a*+*I*)(*b*+*I*) = (*a**b*) +*I*.

(Here one has to check that these definitions are well-defined. Compare coset and quotient group.) The zero-element of *R* / *I* is (0 + *I*) = *I*, and the multiplicative identity is (1 + *I*).

The map *p* from *R* to *R* / *I* defined by *p*(*a*) = *a* + *I* is a surjective ring homomorphism, sometimes called the * natural quotient map* or the

*.*

**canonical homomorphism**Examples

The quotient ring

*R*/ {0} is naturally isomorphic to*R*, and*R*/*R*is the zero ring {0}, since, by our definition, for any*r*in*R*, we have that [*r*] =*r*+ {0} := {*r*+*b*:*b*∈ {0}} (where {0} is the zero ring), which is isomorphic to*R*itself. This fits with the rule of thumb that the larger the ideal*I*, the smaller the quotient ring*R*/*I*. If*I*is a proper ideal of*R*, i.e.,*I*≠*R*, then*R*/*I*is not the zero ring.Consider the ring of integers

**Z**and the ideal of even numbers, denoted by 2**Z**. Then the quotient ring**Z**/ 2**Z**has only two elements, zero for the even numbers and one for the odd numbers; applying the definition, [*z*] =*z*+ 2**Z**:= {*z*+ 2*y*: 2*y*∈ 2**Z**}, where 2**Z**is the ideal of even numbers. It is naturally isomorphic to the finite field with two elements,**F**2. Intuitively: if you think of all the even numbers as 0, then every integer is either 0 (if it is even) or 1 (if it is odd and therefore differs from an even number by 1). Modular arithmetic is essentially arithmetic in the quotient ring**Z**/*n***Z**(which has*n*elements).Now consider the ring

**R**[*X*] of polynomials in the variable*X*with real coefficients, and the ideal*I*= (*X*2 + 1) consisting of all multiples of the polynomial*X*2 + 1. The quotient ring**R**[*X*] / (*X*2 + 1) is naturally isomorphic to the field of complex numbers**C**, with the class [*X*] playing the role of the imaginary unit*i*. The reason is that we "forced"*X*2 + 1 = 0, i.e.*X*2 = −1, which is the defining property of*i*.Generalizing the previous example, quotient rings are often used to construct field extensions. Suppose

*K*is some field and*f*is an irreducible polynomial in*K*[*X*]. Then*L*=*K*[*X*] / (*f*) is a field whose minimal polynomial over*K*is*f*, which contains*K*as well as an element*x*=*X*+ (*f*).One important instance of the previous example is the construction of the finite fields. Consider for instance the field

**F**3 =**Z**/ 3**Z**with three elements. The polynomial*f*(*X*) =*X*2 + 1 is irreducible over**F**3 (since it has no root), and we can construct the quotient ring**F**3[*X*] / (*f*). This is a field with 32 = 9 elements, denoted by**F**9. The other finite fields can be constructed in a similar fashion.The coordinate rings of algebraic varieties are important examples of quotient rings in algebraic geometry. As a simple case, consider the real variety

*V*= {(*x*,*y*) |*x*2 =*y*3 } as a subset of the real plane**R**2. The ring of real-valued polynomial functions defined on*V*can be identified with the quotient ring**R**[*X*,*Y*] / (*X*2 −*Y*3), and this is the coordinate ring of*V*. The variety*V*is now investigated by studying its coordinate ring.Suppose

*M*is a C∞-manifold, and*p*is a point of*M*. Consider the ring*R*= C∞(*M*) of all C∞-functions defined on*M*and let*I*be the ideal in*R*consisting of those functions*f*which are identically zero in some neighborhood*U*of*p*(where*U*may depend on*f*). Then the quotient ring*R*/*I*is the ring of germs of C∞-functions on*M*at*p*.Consider the ring

*F*of finite elements of a hyperreal fieldn* <**R**. It consists of all hyperreal numbers differing from a standard real by an infinitesimal amount, or equivalently: of all hyperreal numbers*x*for which a standard integer*n*with −*x*<*n*exists. The set*I*of all infinitesimal numbers in ***R**, together with 0, is an ideal in*F*, and the quotient ring*F*/*I*is isomorphic to the real numbers**R**. The isomorphism is induced by associating to every element*x*of*F*the standard part of*x*, i.e. the unique real number that differs from*x*by an infinitesimal. In fact, one obtains the same result, namely**R**, if one starts with the ring*F*of finite hyperrationals (i.e. ratio of a pair of hyperintegers), see construction of the real numbers.

Alternative complex planes

The quotients **R**[*X*] / (*X*), **R**[X] / (*X* + 1), and **R**[*X*] / (*X* − 1) are all isomorphic to **R** and gain little interest at first. But note that **R**[*X*] / (*X*2) is called the dual number plane in geometric algebra. It consists only of linear binomials as "remainders" after reducing an element of **R**[*X*] by *X*2. This alternative complex plane arises as a subalgebra whenever the algebra contains a real line and a nilpotent.

Furthermore, the ring quotient **R**[*X*] / (*X*2 − 1) does split into **R**[*X*] / (*X* + 1) and **R**[*X*] / (*X* − 1), so this ring is often viewed as the direct sum **R** ⊕ **R**.
Nevertheless, an alternative complex number *z* = *x* + *y* j is suggested by j as a root of *X*2 − 1, compared to i as root of *X*2 + 1 = 0. This plane of split-complex numbers normalizes the direct sum *R'* ⊕ **R** by providing a basis {1, j} for 2-space where the identity of the algebra is at unit distance from the zero. With this basis a unit hyperbola may be compared to the unit circle of the ordinary complex plane.

Quaternions and alternatives

Suppose *X* and *Y* are two, non-commuting, indeterminates and form the free algebra **R**⟨*X*, *Y*⟩. Then Hamilton’s quaternions of 1843 can be cast as

If *Y*2 − 1 is substituted for *Y*2 + 1, then one obtains the ring of split-quaternions. Substituting minus for plus in *both* the quadratic binomials also results in split-quaternions. The anti-commutative property *YX* = −*XY* implies that *XY* has as its square

- (

*XY*)(

*XY*) =

*X*(

*YX*)

*Y*= −

*X*(

*XY*)

*Y*= −

*XXYY*= −1.

The three types of biquaternions can also be written as quotients by use of the free algebra with three indeterminates **R**⟨*X*, *Y*, *Z*⟩ and constructing appropriate ideals.

Properties

Clearly, if *R* is a commutative ring, then so is *R* / *I*; the converse however is not true in general.

The natural quotient map *p* has *I* as its kernel; since the kernel of every ring homomorphism is a two-sided ideal, we can state that two-sided ideals are precisely the kernels of ring homomorphisms.

The intimate relationship between ring homomorphisms, kernels and quotient rings can be summarized as follows: *the ring homomorphisms defined on R / I are essentially the same as the ring homomorphisms defined on R that vanish (i.e. are zero) on I*. More precisely, given a two-sided ideal *I* in *R* and a ring homomorphism *f* : *R* → *S* whose kernel contains *I*, there exists precisely one ring homomorphism *g* : *R* / *I* → *S* with *gp* = *f* (where *p* is the natural quotient map). The map *g* here is given by the well-defined rule *g*([*a*]) = *f*(*a*) for all *a* in *R*. Indeed, this universal property can be used to *define* quotient rings and their natural quotient maps.

As a consequence of the above, one obtains the fundamental statement: every ring homomorphism *f* : *R* → *S* induces a ring isomorphism between the quotient ring *R* / ker(*f*) and the image im(*f*). (See also: fundamental theorem on homomorphisms.)

The ideals of *R* and *R* / *I* are closely related: the natural quotient map provides a bijection between the two-sided ideals of *R* that contain *I* and the two-sided ideals of *R* / *I* (the same is true for left and for right ideals). This relationship between two-sided ideal extends to a relationship between the corresponding quotient rings: if *M* is a two-sided ideal in *R* that contains *I*, and we write *M* / *I* for the corresponding ideal in *R* / *I* (i.e. *M* / *I* = *p*(*M*)), the quotient rings *R* / *M* and (*R* / *I*) / (*M* / *I*) are naturally isomorphic via the (well-defined!) mapping *a* + *M* ↦ (*a* + *I*) + *M* / *I*.

In commutative algebra and algebraic geometry, the following statement is often used: If *R* ≠ {0} is a commutative ring and *I* is a maximal ideal, then the quotient ring *R* / *I* is a field; if *I* is only a prime ideal, then *R* / *I* is only an integral domain. A number of similar statements relate properties of the ideal *I* to properties of the quotient ring *R* / *I*.

The Chinese remainder theorem states that, if the ideal *I* is the intersection (or equivalently, the product) of pairwise coprime ideals *I*1, ..., *Ik*, then the quotient ring *R* / *I* is isomorphic to the product of the quotient rings *R* / *Ip*, *p* = 1, ..., *k*.

For algebras over a ring

An associative algebra *A* over a commutative ring *R* is a ring itself. If *I* is an ideal in *A* (closed under *R*-multiplication), then *A* / *I* inherits the structure of an algebra over *R* and is the **quotient algebra**.

See also

Associated graded ring

Residue field

Goldie's theorem

Quotient module

## References

*Structure of Rings*(revised ed.). American Mathematical Soc. ISBN 0-821-87470-5.

*Abstract Algebra*(3rd ed.). John Wiley & Sons. ISBN 0-471-43334-9.

*Algebra*. Graduate Texts in Mathematics. Springer. ISBN 0-387-95385-X.