Redirected from Divisibility
In mathematics, a divisor of an integer n, also called a factor of n, is an integer which evenly divides n without leaving a remainder. For example, 7 is a divisor of 42 because 42/7 = 6. We also say 42 is divisible by 7 or 7 divides 42 and we usually write 7  42. Divisors can be positive or negative. The positive divisors of 42 are {1, 2, 3, 6, 7, 14, 21, 42}.
Some special cases: 1 and 1 are divisors of every integer, and every integer is a divisor of 0.
A positive divisor of n which is different from n is called a proper divisor. An integer n > 1 whose only proper divisor is 1 is called a prime number.
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.
The total number of positive divisors of n is a multiplicative function d(n) (e.g. d(42)=8). The sum of the positive divisors of n is another multiplicative function σ(n) (e.g. σ(42)=96).
The relation  of divisibility turns the set N of nonnegative integers into a partially ordered set, in fact into a complete distributive lattice. The largest element of this lattice is 0 and the smallest one is 1. The meet operation ^ is given by the greatest common divisor and the join operation v by the least common multiple. This lattice is isomorphic to the dual of the lattice of subgroups of the infinite cyclic group Z.
There are some rules which allow to recognize small divisors of a number from the number's decimal digits:
One can talk about the concept of divisibility in any integral domain. Please see that article for the definitions in that setting.
In algebraic geometry, the word "divisor" is used to mean something rather different. Divisors are a generalization of subvarieties[?] of algebraic varieties; two different generalizations are in common use, Cartier divisors[?] and Weil divisors[?]. The concepts agree on nonsingular varieties over algebraically closed fields. Any Weil divisor is a locally finite linear combination of irreducible subvarieties of codimension one. To every Cartier divisor D there is an associated line bundle[?] denoted by [D], and the sum of divisors corresponds to tensor product of line bundles.
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