Quotient ring

Short description: Reduction of a ring by one of its ideals

In ring theory, a branch of abstract algebra, a quotient ring, also known as factor ring, difference ring or residue class ring, is a construction quite similar to the quotient group in group theory and to the quotient space in linear algebra. It is a specific example of a quotient, as viewed from the general setting of universal algebra. Starting with a ring $R$ and a two-sided ideal $I$ in $R$, a new ring, the quotient ring $R/I$, is constructed, whose elements are the cosets of $I$ in $R$ subject to special $+$ and $\cdot$ operations. (Quotient ring notation almost always uses a fraction slash "$/$"; stacking the ring over the ideal using a horizontal line as a separator is uncommon and generally avoided.)

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 $\sim$ on $R$ as follows:

$a\sim b$ if and only if $a-b$ is in $I$.

Using the ideal properties, it is not difficult to check that $\sim$ is a congruence relation. In case $a\sim b$, we say that $a$ and $b$ are congruent modulo $I$ (for example, $1$ and $3$ are congruent modulo $2$ as their difference is an element of the ideal $2\mathbb{\mathbb{Z}}$, the even integers). The equivalence class of the element $a$ in $R$ is given by: $$\left[a\right]=\bar{a}=a+I:=\{a+r:r\in I\}$$This equivalence class is also sometimes written as $amodI$ 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

• $(a+I)+(b+I)=(a+b)+I$;
• $(a+I)(b+I)=(ab)+I$.

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

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, natural projection 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 $\left[r\right]=r+R=\{r+b:b\in R\}$, which equals $R$ 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., $I\ne R$, then $R/I$ is not the zero ring.
• Consider the ring of integers $\mathbb{\mathbb{Z}}$ and the ideal of even numbers, denoted by $2\mathbb{\mathbb{Z}}$. Then the quotient ring $\mathbb{\mathbb{Z}}/2\mathbb{\mathbb{Z}}$ has only two elements, the coset $0+2\mathbb{\mathbb{Z}}$ consisting of the even numbers and the coset $1+2\mathbb{\mathbb{Z}}$ consisting of the odd numbers; applying the definition, $\left[z\right]=z+2\mathbb{\mathbb{Z}}=\{z+2y:2y\in 2\mathbb{\mathbb{Z}}\}$, where $2\mathbb{\mathbb{Z}}$ 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 $\mathbb{\mathbb{Z}}/n\mathbb{\mathbb{Z}}$ (which has $n$ elements).
• Now consider the ring of polynomials in the variable $X$ with real coefficients, $\mathbb{ℝ}[X]$, and the ideal $I=(X^2+1)$ consisting of all multiples of the polynomial $X^2+1$. The quotient ring $\mathbb{ℝ}[X]/(X^2+1)$ is naturally isomorphic to the field of complex numbers $\mathbb{ℂ}$, 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 $i$. Since any integer exponent of $i$ must be either $\pm i$ or $\pm 1$, that means all possible polynomials essentially simplify to the form $a+bi$. (To clarify, the quotient ring $\mathbb{ℝ}[X]/(X^2+1)$ is actually naturally isomorphic to the field of all linear polynomials $aX+b;a,b\in \mathbb{ℝ}$, where the operations are performed modulo $X^2+1$. In return, we have $X^2=-1$, and this is matching $X$ to the imaginary unit in the isomorphic field of complex numbers.)
• 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 $F_3=\mathbb{\mathbb{Z}}/3\mathbb{\mathbb{Z}}$ 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 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 ${\mathbb{ℝ}}^2$. The ring of real-valued polynomial functions defined on $V$ can be identified with the quotient ring $\mathbb{ℝ}[X,Y]/(X^2-Y^3)$, and this is the coordinate ring of $V$. The variety $V$ is now investigated by studying its coordinate ring.
• Suppose $M$ is a ${\mathbb{ℂ}}^{\mathrm{\infty }}$-manifold, and $p$ is a point of $M$. Consider the ring $R={\mathbb{ℂ}}^{\mathrm{\infty }}(M)$ of all ${\mathbb{ℂ}}^{\mathrm{\infty }}$-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 ${\mathbb{ℂ}}^{\mathrm{\infty }}$-functions on $M$ at $p$.
• Consider the ring $F$ of finite elements of a hyperreal field ${*}^{}\mathbb{ℝ}$. 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 ${*}^{}\mathbb{ℝ}$, 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 of hyperintegers), see construction of the real numbers.

Variations of complex planes

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

Furthermore, the ring quotient $\mathbb{ℝ}[X]/(X^2-1)$ does split into $\mathbb{ℝ}[X]/(X+1)$ and $\mathbb{ℝ}[X]/(X-1)$, so this ring is often viewed as the direct sum $\mathbb{ℝ}\oplus \mathbb{ℝ}$. Nevertheless, a variation on complex numbers $z=x+yj$ is suggested by $j$ as a root of $X^2-1=0$, compared to $i$ as root of $X^2+1=0$. This plane of split-complex numbers normalizes the direct sum $\mathbb{ℝ}\oplus \mathbb{ℝ}$ 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 variations

Suppose $X$ and $Y$ are two non-commuting indeterminates and form the free algebra $\mathbb{ℝ}⟨X,Y⟩$. Then Hamilton's quaternions of 1843 can be cast as: $$\mathbb{ℝ}⟨X,Y⟩/(X^2+1,Y^2+1,XY+YX)$$

If $Y^2-1$ is substituted for $Y^2+1$, then one obtains the ring of split-quaternions. The anti-commutative property $YX=-XY$ implies that $XY$ has as its square: $$(XY)(XY)=X(YX)Y=-X(XY)Y=-(XX)(YY)=-(-1)(+1)=+1$$

Substituting minus for plus in both the quadratic binomials also results in split-quaternions.

The three types of biquaternions can also be written as quotients by use of the free algebra with three indeterminates $\mathbb{ℝ}⟨X,Y,Z⟩$ 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 $f:R\to S$ whose kernel contains $I$, there exists precisely one ring homomorphism $g:R/I\to S$ 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 $1R$. 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 $f:R\to S$ induces a ring isomorphism between the quotient ring $R/\ker (f)$ and the image $\mathrm{i}\mathrm{m}(f)$. (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 $a+M\mapsto (a+I)+M/I$.

The following facts prove useful in commutative algebra and algebraic geometry: for $R\ne \{0\}$ commutative, $R/I$ is a field if and only if $I$ is a maximal ideal, while $R/I$ is an integral domain if and only if $I$ is a prime ideal. 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 $I_1,\dots ,I_k$, then the quotient ring $R/I$ is isomorphic to the product of the quotient rings $R/I_n,n=1,\dots ,k$.

For algebras over a ring

An associative algebra $A$ over a commutative ring $R$ is itself a ring. If $I$ is an ideal in $A$ (closed under $A$-multiplication: $AI\subseteq I$), 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

Further references

• F. Kasch (1978) Moduln und Ringe, translated by DAR Wallace (1982) Modules and Rings, Academic Press, page 33.
• Neal H. McCoy (1948) Rings and Ideals, §13 Residue class rings, page 61, Carus Mathematical Monographs #8, Mathematical Association of America.
• Joseph Rotman (1998). Galois Theory (2nd ed.). Springer. pp. 21–23. ISBN 0-387-98541-7. 
• B.L. van der Waerden (1970) Algebra, translated by Fred Blum and John R Schulenberger, Frederick Ungar Publishing, New York. See Chapter 3.5, "Ideals. Residue Class Rings", pp. 47–51.

External links

• Hazewinkel, Michiel, ed. (2001), "Quotient ring", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4, https://www.encyclopediaofmath.org/index.php?title=p/q076920 
• Ideals and factor rings from John Beachy's Abstract Algebra Online