Divisor

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

Definition

If ${\displaystyle m}$ and ${\displaystyle n}$ are nonzero integers, and more generally, nonzero elements of an integral domain, it is said that ${\displaystyle m}$ divides ${\displaystyle n}$, ${\displaystyle m}$ is a divisor of ${\displaystyle n,}$ or ${\displaystyle n}$ is a multiple of ${\displaystyle m,}$ and this is written as

${\displaystyle m\mid n,}$

if there exists an integer ${\displaystyle k}$, or an element ${\displaystyle k}$ of the integral domain, such that ${\displaystyle mk=n}$.[1]

This definition is sometimes extended to include zero.[2] This does not add much to the theory, as 0 does not divide any other number, and every number divides 0. On the other hand, excluding zero from the definition simplifies many statements. Also, in ring theory, an element a is called a "zero divisor" only if it is nonzero and ab = 0 for a nonzero element b. Thus, there are no zero divisors among the integers (and by definition no zero divisors in an integral domain).

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 [3]) . 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 ${\displaystyle 7\times 6=42}$, 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:

Further notions and facts

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 ${\displaystyle a\mid (b-c)}$.[4] 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 gcd${\displaystyle (a,b)=1}$, then ${\displaystyle a\mid c}$. 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 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,[5]

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

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 small and "abnormally large" divisors.

In abstract algebra

In definitions that include 0, the relation of divisibility turns the set ${\displaystyle \mathbb {N} }$ 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 ${\displaystyle \mathbb {Z} }$.

Notes

1. for instance, Sims 1984, p. 42 or Durbin 1992, p. 61
2. Herstein 1986, p. 26
3. FoCaLiZe and Dedukti to the Rescue for Proof Interoperability by Raphael Cauderlier and Catherine Dubois
4. ${\displaystyle a\mid b,\,a\mid c\Rightarrow b=ja,\,c=ka\Rightarrow b+c=(j+k)a\Rightarrow a\mid (b+c)}$. Similarly, ${\displaystyle a\mid b,\,a\mid c\Rightarrow b=ja,\,c=ka\Rightarrow b-c=(j-k)a\Rightarrow a\mid (b-c)}$
5. Hardy, G. H.; Wright, E. M. (April 17, 1980). An Introduction to the Theory of Numbers. Oxford University Press. p. 264. ISBN 0-19-853171-0.

References

• Durbin, John R. (1992). Modern Algebra: An Introduction (3rd ed.). New York: Wiley. ISBN 0-471-51001-7.
• Richard K. Guy, Unsolved Problems in Number Theory (3rd ed), Springer Verlag, 2004 ISBN 0-387-20860-7; section B.
• Herstein, I. N. (1986), Abstract Algebra, New York: Macmillan Publishing Company, ISBN 0-02-353820-1
• 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