Divisor

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

${\displaystyle m\mid n.}$

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 ${\displaystyle m\not \mid n}$.[1][2]

Usually, m is required to be nonzero, but n is allowed to be zero. With this convention, ${\displaystyle m\mid 0}$ for every nonzero integer m.[1][2] Some definitions omit the requirement that ${\displaystyle m}$ be nonzero.[3]

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[4]). 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.

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 ${\displaystyle 7\times 6=42}$, so we can say ${\displaystyle 7\mid 42}$. 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, ${\displaystyle A=\{1,2,3,4,5,6,10,12,15,20,30,60\}}$, partially ordered by divisibility, has the Hasse diagram:

There are some elementary rules:

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

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

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

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

An integer ${\displaystyle n>1}$ 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 ${\displaystyle n}$ is a product of prime divisors of ${\displaystyle n}$ raised to some power. This is a consequence of the fundamental theorem of arithmetic.

A number ${\displaystyle n}$ 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 ${\displaystyle n}$, and abundant if this sum exceeds ${\displaystyle n}$.

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

If the prime factorization of ${\displaystyle n}$ is given by

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

then the number of positive divisors of ${\displaystyle n}$ is

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

and each of the divisors has the form

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

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

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

Also,[6]

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

where ${\displaystyle \gamma }$ 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 ${\displaystyle \ln n}$. However, this is a result from the contributions of numbers with "abnormally many" divisors.

• 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

• 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