${\mathbb{Z}}_{n}$

Let $n\in\mathbb{Z}$ . An equivalence relation, called congruence (http://planetmath.org/Congruent2), can be defined on $\mathbb{Z}$ by $a\equiv b\operatorname{mod}n$ iff $n$ divides $b-a$ . Note first of all that $a\equiv b\operatorname{mod}n$ iff $a\equiv b\operatorname{mod}|n|$ . Thus, without loss of generality, only nonnegative $n$ need be considered. Secondly, note that the case $n=0$ is not very interesting. If $a\equiv b\operatorname{mod}0$ , then $0$ divides $b-a$ , which occurs exactly when $a=b$ . In this case, the set of all equivalence classes can be identified with $\mathbb{Z}$ . Thus, only positive $n$ need be considered. The set of all equivalence classes of $\mathbb{Z}$ under the given equivalence relation is called ${\mathbb{Z}}_{n}$ .

Some mathematicians consider the notation ${\mathbb{Z}}_{n}$ to be archaic and somewhat confusing. This matter of notation is most considerable when $n=p$ for some prime (http://planetmath.org/PrimeNumber) $p$ , as ${\mathbb{Z}}_{p}$ is used to refer to the $p$ -adic integers (http://planetmath.org/MathbbZ_p). To avoid this confusion, some mathematicians use the notation $\mathbb{Z}/n\mathbb{Z}$ instead of ${\mathbb{Z}}_{n}$ . On the other hand, the notation ${\mathbb{Z}}_{n}$ should not cause confusion when $n$ is not prime, and is an intuitive shorthand way to write $\mathbb{Z}/n\mathbb{Z}$ . Thus, others use ${\mathbb{F}}_{p}$ when $n=p$ for some prime $p$ and ${\mathbb{Z}}_{n}$ otherwise. (The explanation of the usage of $\mathbb{F}_{p}$ will come later.) Still others, especially those who are unfamiliar with the , use the notation ${\mathbb{Z}}_{n}$ exclusively. (In this entry, the notation ${\mathbb{Z}}_{n}$ is used exclusively, though it is highly recommended to use another notation when $n=p$ for some prime $p$ .)

One usually identifies an element of ${\mathbb{Z}}_{n}$ (which is technically a class (http://planetmath.org/EquivalenceClass), ) with the unique element $r$ in the class such that $0\leq r<n$ . One can use the division algorithm to establish that, for each class, an $r$ as described exists uniquely. (The set of all $r$ ’s as described is an example of a residue system.) Thus, the sets ${\mathbb{Z}}_{n}$ are finite with exactly $n$ elements. Addition and multiplication operations can also be defined on ${\mathbb{Z}}_{n}$ in a natural way that corresponds to the operations on $\mathbb{Z}$ . Under these operations, ${\mathbb{Z}}_{n}$ is a commutative ring with identity (http://planetmath.org/MultiplicativeIdentity) as well as a cyclic ring with behavior $1$ . When $n=p$ for some prime $p$ , ${\mathbb{Z}}_{n}$ is a field. In this case, the notation ${\mathbb{F}}_{p}$ highlights the fact that the is a field. When $n$ is composite, ${\mathbb{Z}}_{n}$ has zero divisors and thus is neither a field nor an integral domain. Also note that ${\mathbb{Z}}_{1}$ is a zero ring, since all integers are equivalent (http://planetmath.org/Equivalent), yielding only one equivalence class.

The $n$ in both ${\mathbb{Z}}_{n}$ and $a\equiv b\operatorname{mod}n$ is called the modulus. Performing computations such as addition, subtraction, multiplication, and taking exponents (http://planetmath.org/Exponent2) in one of the rings ${\mathbb{Z}}_{n}$ is called modular arithmetic.

Title	${\mathbb{Z}}_{n}$
Canonical name	mathbbZn
Date of creation	2013-03-22 15:58:10
Last modified on	2013-03-22 15:58:10
Owner	Wkbj79 (1863)
Last modified by	Wkbj79 (1863)
Numerical id	25
Author	Wkbj79 (1863)
Entry type	Definition
Classification	msc 13-00
Classification	msc 13M05
Classification	msc 11-00
Synonym	integers mod n
Related topic	ResidueSystems
Related topic	MathbbZ
Related topic	CyclicRingsThatAreIsomorphicToKmathbbZ_kn
Related topic	Congruences
Related topic	EquivalenceRelation
Defines	modulus
Defines	modular arithmetic

ℤn

${\mathbb{Z}}_{n}$