# Divisibility (ring theory)

In mathematics, the notion of a divisor originally arose within the context of arithmetic of whole numbers. With the development of abstract rings, of which the integers are the archetype, the original notion of divisor found a natural extension.

Divisibility is a useful concept for the analysis of the structure of commutative rings because of its relationship with the ideal structure of such rings.

## Definition

Let R be a ring, and let a and b be elements of R. If there exists an element x in R with ax = b, one says that a is a left divisor of b in R and that b is a right multiple of a. Similarly, if there exists an element y in R with ya = b, one says that a is a right divisor of b and that b is a left multiple of a. One says that a is a two-sided divisor of b if it is both a left divisor and a right divisor of b; in this case, it is not necessarily true that (using the previous notation) x=y, only that both some x and some y which each individually satisfy the previous equations in R exist in R.

When R is commutative, a left divisor, a right divisor and a two-sided divisor coincide, so in this context one says that a is a divisor of b, or that b is a multiple of a, and one writes $a\mid b$ . Elements a and b of an integral domain are associates if both $a\mid b$  and $b\mid a$ . The associate relationship is an equivalence relation on R, and hence divides R into disjoint equivalence classes.

Notes: These definitions make sense in any magma R, but they are used primarily when this magma is the multiplicative monoid of a ring.

## Properties

Statements about divisibility in a commutative ring $R$  can be translated into statements about principal ideals. For instance,

• One has $a\mid b$  if and only if $(b)\subseteq (a)$ .
• Elements a and b are associates if and only if $(a)=(b)$ .
• An element u is a unit if and only if u is a divisor of every element of R.
• An element u is a unit if and only if $(u)=R$ .
• If $a=bu$  for some unit u, then a and b are associates. If R is an integral domain, then the converse is true.
• Let R be an integral domain. If the elements in R are totally ordered by divisibility, then R is called a valuation ring.

In the above, $(a)$  denotes the principal ideal of $R$  generated by the element $a$ .

## Zero as a divisor, and zero divisors

• Some authors require a to be nonzero in the definition of divisor, but this causes some of the properties above to fail.
• If one interprets the definition of divisor literally, every a is a divisor of 0, since one can take x = 0. Because of this, it is traditional to abuse terminology by making an exception for zero divisors: one calls an element a in a commutative ring a zero divisor if there exists a nonzero x such that ax = 0.