- Quotient ring
In

mathematics a**quotient ring**, also known as**factor ring**or**residue class ring**, is a construction inring theory , quite similar to thefactor group s ofgroup theory and the quotient spaces oflinear algebra . One starts with a ring "R" and atwo-sided ideal "I" in "R", and constructs a new ring, the quotient ring "R"/"I", essentially by requiring that all elements of "I" be zero. Intuitively, the quotient ring "R"/"I" is a "simplified version" of "R" where the elements of "I" are "ignored".Quotient rings are distinct from the so-called 'quotient field', or

field of fractions , of anintegral 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 "b" − "a" is in "I". Using the ideal properties, it is not difficult to check that ~ is acongruence relation .In case "a" ~ "b", we say that "a" and "b" are "congruentmodulo " "I".Theequivalence 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 arewell-defined . Comparecoset andquotient 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 most extreme examples of quotient rings are provided by

modding out the most extreme ideals, {0} and "R" itself. "R"/{0} isnaturally isomorphic to "R", and "R"/"R" is thetrivial ring {0}. This fits with the general rule of thumb that "the smaller the ideal I, the larger the quotient ring R/I". If "I" is a proper ideal of "R", i.e. "I" ≠ "R", then "R"/"I" won't be the trivial ring.*Consider the ring of

integer s**Z**and the ideal ofeven number s, denoted by 2**Z**. Then the quotient ring**Z**/2**Z**has only two elements, one for the even numbers and one for the odd numbers. It is naturally isomorphic to thefinite 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 ofcomplex number s**C**, with the class ["X"] playing the role of theimaginary unit "i". The reason: 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 extension s. Suppose "K" is some field and "f" is anirreducible polynomial in "K" ["X"] . Then "L" = "K" ["X"] /("f") is a field which contains "K" as well as an element "x" = "X" + ("f") whoseminimal polynomial over "K" is "f".*One important instance of the previous example is the construction of the

finite field s. 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 3^{2}=9 elements, denoted by**F**_{9}. The other finite fields can be constructed in a similar fashion.*The

coordinate ring s of algebraic varieties are important examples of quotient rings inalgebraic 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 field *

**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 −"n" < "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 $mathbb\{R\}$. 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 $mathbb\{R\}$, if one starts with the ring "F" of finite hyperrationals (i.e. ratio of a pair ofhyperinteger s), seeconstruction 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 thedual 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 frequently enough to accent its existence.Furthermore, the ring quotient

**R**["X"] /("X"^{2}− 1) does split into**R**["X"] /("X" + 1) and**R**["X"] /("X" − 1), so thissplit-complex number ring is often viewed as thedirect sum **R**$oplus$**R**.Nevertheless, a complex number structure based on a hyperbola is brought in. The planar linear algebra ofsqueeze mapping , a.k.a. hyperbolic rotation, fits naturally. The parallel with ordinary complex number representation of circular rotation is a part of split-complex number assignments and arithmetic.**Quaternions and alternatives**Hamilton’s

quaternion s of 1843 can be cast as**R**["X","Y"] /("X"^{2}+ 1, "Y"^{2}+ 1, "XY" + "YX"). If "Y"^{2}− 1 is substituted for "Y"^{2}+ 1, then one obtains the ring ofsplit-quaternion s. Substituting minus for plus in "both" the quadratic binomials also results in split-quaternions: Theanti-commutative property YX = −XY implies that XY has for its square: ("XY")("XY") = "X"("YX")"X" = −"X"("XY")"Y" = − "XXYY" = −1.

The three types of

biquaternion s can also be written as quotients by conscripting the three-indeterminate ring**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", then 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 aring 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 andalgebraic geometry , the following statement is often used: If "R" ≠ {0} is acommutative ring and "I" is amaximal ideal , then the quotient ring "R"/"I" is a field; if "I" is only aprime ideal , then "R"/"I" is only anintegral 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}",...,"I_{k}", then the quotient ring "R"/"I" is isomorphic to the product of the quotient rings "R"/"I_{p}" , "p"=1,...,"k".**ee also***

Residue field **External links*** [

*http://www.math.niu.edu/~beachy/aaol/rings.html#ideals Ideals and factor rings*] from John Beachy's "Abstract Algebra Online"

*

*Wikimedia Foundation.
2010.*

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