Divisor

This article is about an integer that is a factor of another integer. For a number used to divide another number in a division operation, see Division (mathematics). For other uses, see Divisor (disambiguation).

"Divisible" redirects here. For divisibility of groups, see Divisible group.

The divisors of 10 illustrated with Cuisenaire rods: 1, 2, 5, and 10

In mathematics, a divisor of an integer $n$, also called a factor of $n$, is an integer $m$ that may be multiplied by some integer to produce $n$. In this case, one also says that $n$ is a multiple of $m.$ An integer $n$ is divisible or evenly divisible by another integer $m$ if $m$ is a divisor of $n$; this implies dividing $n$ by $m$ leaves no remainder.

Definition

An integer n is divisible by a nonzero integer m if there exists an integer k such that $n=km$. This is written as

$m\mid n.$

Other ways of saying the same thing are that m divides n, m is a divisor of n, m is a factor of n, and n is a multiple of m. If m does not divide n, then the notation is $m\not \mid n$.

Usually, m is required to be nonzero, but n is allowed to be zero. With this convention, $m\mid 0$ for every nonzero integer m. Some definitions omit the requirement that $m$ be nonzero.

General

Divisors can be negative as well as positive, although sometimes the term is restricted to positive divisors. For example, there are six divisors of 4; they are 1, 2, 4, −1, −2, and −4, but only the positive ones (1, 2, and 4) would usually be mentioned.

1 and −1 divide (are divisors of) every integer. Every integer (and its negation) is a divisor of itself. Integers divisible by 2 are called even, and integers not divisible by 2 are called odd.

1, −1, n and −n are known as the trivial divisors of n. A divisor of n that is not a trivial divisor is known as a non-trivial divisor (or strict divisor). A nonzero integer with at least one non-trivial divisor is known as a composite number, while the units −1 and 1 and prime numbers have no non-trivial divisors.

There are divisibility rules that allow one to recognize certain divisors of a number from the number's digits.

Examples

Plot of the number of divisors of integers from 1 to 1000. Prime numbers have exactly 2 divisors, and highly composite numbers are in bold.

• 7 is a divisor of 42 because $7\times 6=42$, so we can say $7\mid 42$. It can also be said that 42 is divisible by 7, 42 is a multiple of 7, 7 divides 42, or 7 is a factor of 42.
• The non-trivial divisors of 6 are 2, −2, 3, −3.
• The positive divisors of 42 are 1, 2, 3, 6, 7, 14, 21, 42.
• The set of all positive divisors of 60, $A=\{1,2,3,4,5,6,10,12,15,20,30,60\}$, partially ordered by divisibility, has the Hasse diagram:

Further notions and facts

There are some elementary rules:

• If $a\mid b$ and $b\mid c$, then $a\mid c$, i.e. divisibility is a transitive relation.
• If $a\mid b$ and $b\mid a$, then $a=b$ or $a=-b$.
• If $a\mid b$ and $a\mid c$, then $a\mid (b+c)$ holds, as does $a\mid (b-c)$. However, if $a\mid b$ and $c\mid b$, then $(a+c)\mid b$ does not always hold (e.g. $2\mid 6$ and $3\mid 6$ but 5 does not divide 6).

If $a\mid bc$, and $\gcd(a,b)=1$, then $a\mid c$. This is called Euclid's lemma.

If $p$ is a prime number and $p\mid ab$ then $p\mid a$ or $p\mid b$.

A positive divisor of $n$ which is different from $n$ is called a proper divisor or an aliquot part of $n$. A number that does not evenly divide $n$ but leaves a remainder is sometimes called an aliquant part of $n$.

An integer $n>1$ whose only proper divisor is 1 is called a prime number. Equivalently, a prime number is a positive integer that has exactly two positive factors: 1 and itself.

Any positive divisor of $n$ is a product of prime divisors of $n$ raised to some power. This is a consequence of the fundamental theorem of arithmetic.

A number $n$ is said to be perfect if it equals the sum of its proper divisors, deficient if the sum of its proper divisors is less than $n$, and abundant if this sum exceeds $n$.

The total number of positive divisors of $n$ is a multiplicative function $d(n)$, meaning that when two numbers $m$ and $n$ are relatively prime, then $d(mn)=d(m)\times d(n)$. For instance, $d(42)=8=2\times 2\times 2=d(2)\times d(3)\times d(7)$; the eight divisors of 42 are 1, 2, 3, 6, 7, 14, 21 and 42. However, the number of positive divisors is not a totally multiplicative function: if the two numbers $m$ and $n$ share a common divisor, then it might not be true that $d(mn)=d(m)\times d(n)$. The sum of the positive divisors of $n$ is another multiplicative function $\sigma (n)$ (e.g. $\sigma (42)=96=3\times 4\times 8=\sigma (2)\times \sigma (3)\times \sigma (7)=1+2+3+6+7+14+21+42$). Both of these functions are examples of divisor functions.

If the prime factorization of $n$ is given by

$n=p_{1}^{\nu _{1}}\,p_{2}^{\nu _{2}}\cdots p_{k}^{\nu _{k}}$

then the number of positive divisors of $n$ is

$d(n)=(\nu _{1}+1)(\nu _{2}+1)\cdots (\nu _{k}+1),$

and each of the divisors has the form

$p_{1}^{\mu _{1}}\,p_{2}^{\mu _{2}}\cdots p_{k}^{\mu _{k}}$

where $0\leq \mu _{i}\leq \nu _{i}$ for each $1\leq i\leq k.$

For every natural $n$, $d(n)<2{\sqrt {n}}$.

Also,

$d(1)+d(2)+\cdots +d(n)=n\ln n+(2\gamma -1)n+O({\sqrt {n}}).$

where $\gamma$ is Euler–Mascheroni constant. One interpretation of this result is that a randomly chosen positive integer n has an average number of divisors of about $\ln n$. However, this is a result from the contributions of numbers with "abnormally many" divisors.

In abstract algebra

Ring theory

Main article: Divisibility (ring theory)

Division lattice

Main article: Division lattice

In definitions that include 0, the relation of divisibility turns the set $\mathbb {N}$ of non-negative integers into a partially ordered set: a complete distributive lattice. The largest element of this lattice is 0 and the smallest is 1. The meet operation ∧ is given by the greatest common divisor and the join operation ∨ by the least common multiple. This lattice is isomorphic to the dual of the lattice of subgroups of the infinite cyclic group $\mathbb {Z}$.

See also

• Arithmetic functions
• Euclidean algorithm
• Fraction (mathematics)
• Table of divisors — A table of prime and non-prime divisors for 1–1000
• Table of prime factors — A table of prime factors for 1–1000
• Unitary divisor

References

• Durbin, John R. (2009). Modern Algebra: An Introduction (6th ed.). New York: Wiley. ISBN 978-0470-38443-5.
• Richard K. Guy, Unsolved Problems in Number Theory (3rd ed), Springer Verlag, 2004 ISBN 0-387-20860-7; section B.
• Hardy, G. H.; Wright, E. M. (1960). An Introduction to the Theory of Numbers (4th ed.). Oxford University Press.
• Herstein, I. N. (1986), Abstract Algebra, New York: Macmillan Publishing Company, ISBN 0-02-353820-1
• Niven, Ivan; Zuckerman, Herbert S.; Montgomery, Hugh L. (1991). An Introduction to the Theory of Numbers (5th ed.). John Wiley & Sons. ISBN 0-471-62546-9.
• Øystein Ore, Number Theory and its History, McGraw–Hill, NY, 1944 (and Dover reprints).
• Sims, Charles C. (1984), Abstract Algebra: A Computational Approach, New York: John Wiley & Sons, ISBN 0-471-09846-9