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  m is a divisor of  m is a factor of  and n is a multiple of  If  does not divide , then the notation is
Usually,  is required to be nonzero, but  is allowed to be zero.
With this convention, m \mid 0 for every nonzero integer .
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 divisorFoCaLiZe and Dedukti to the Rescue for Proof Interoperability by  Raphael Cauderlier and Catherine Dubois).
A non-zero 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
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:
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==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).a \mid b,\, a \mid c \Rightarrow b=ja,\, c=ka \Rightarrow b+c=(j+k)a \Rightarrow a \mid (b+c).
Similarly, a \mid b,\, a \mid c \Rightarrow b=ja,\, c=ka \Rightarrow b-c=(j-k)a \Rightarrow 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\mid6 and 3 \mid 6 but 5 does not divide 6).
If a \mid bc, and \gcd(a, b) = 1, then a \mid c.\gcd refers to the greatest common divisor.
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  or an  of n.
A number that does not evenly divide n but leaves a remainder is sometimes called an  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 \le \mu_i \le \nu_i  for each 1 \le i \le 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
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
Euclid's 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
Notes
References
Richard K. Guy, Unsolved Problems in Number Theory (3rd ed), Springer Verlag, 2004 ; section B.
Øystein Ore, Number Theory and its History, McGraw–Hill, NY, 1944 (and Dover reprints).
