Zero divisor

In abstract algebra, an element a of a ring R is called a left zero divisor if there exists a nonzero x such that ax = 0, or equivalently if the map from R to R that sends x to ax is not injective.[a] Similarly, an element a of a ring is called a right zero divisor if there exists a nonzero y such that ya = 0. This is a partial case of divisibility in rings. An element that is a left or a right zero divisor is simply called a zero divisor. An element a that is both a left and a right zero divisor is called a two-sided zero divisor (the nonzero x such that ax = 0 may be different from the nonzero y such that ya = 0). If the ring is commutative, then the left and right zero divisors are the same.

An element of a ring that is not a zero divisor is called regular, or a non-zero-divisor. A zero divisor that is nonzero is called a nonzero zero divisor or a nontrivial zero divisor. If there are no nontrivial zero divisors in R, then R is a domain.

Examples

• In the ring $\mathbb /4\mathbb$ , the residue class ${\overline }$ is a zero divisor since ${\overline }\times {\overline }={\overline }={\overline }$ .
• The only zero divisor of the ring $\mathbb$ of integers is $0$ .
• A nilpotent element of a nonzero ring is always a two-sided zero divisor.
• An idempotent element $e\neq 1$ of a ring is always a two-sided zero divisor, since $e(1-e)=0=(1-e)e$ .
• Examples of zero divisors in the ring of $2\times 2$ matrices (over any nonzero ring) are shown here:
${\begin1&1\\2&2\end}{\begin1&1\\-1&-1\end}={\begin-2&1\\-2&1\end}{\begin1&1\\2&2\end}={\begin0&0\\0&0\end},$ ${\begin1&0\\0&0\end}{\begin0&0\\0&1\end}={\begin0&0\\0&1\end}{\begin1&0\\0&0\end}={\begin0&0\\0&0\end}$ .
• A direct product of two or more nonzero rings always has nonzero zero divisors. For example, in $R_\times R_$ with each $R_$ nonzero, $(1,0)(0,1)=(0,0)$ , so $(1,0)$ is a zero divisor.

One-sided zero-divisor

• Consider the ring of (formal) matrices ${\beginx&y\\0&z\end}$ with $x,z\in \mathbb$ and $y\in \mathbb /2\mathbb$ . Then ${\beginx&y\\0&z\end}{\begina&b\\0&c\end}={\beginxa&xb+yc\\0&zc\end}$ and ${\begina&b\\0&c\end}{\beginx&y\\0&z\end}={\beginxa&ya+zb\\0&zc\end}$ . If $x\neq 0\neq z$ , then ${\beginx&y\\0&z\end}$ is a left zero divisor iff $x$ is even, since ${\beginx&y\\0&z\end}{\begin0&1\\0&0\end}={\begin0&x\\0&0\end}$ , and it is a right zero divisor iff $z$ is even for similar reasons. If either of $x,z$ is $0$ , then it is a two-sided zero-divisor.
• Here is another example of a ring with an element that is a zero divisor on one side only. Let $S$ be the set of all sequences of integers $(a_,a_,a_,...)$ . Take for the ring all additive maps from $S$ to $S$ , with pointwise addition and composition as the ring operations. (That is, our ring is $\mathrm (S)$ , the endomorphism ring of the additive group $S$ .) Three examples of elements of this ring are the right shift $R(a_,a_,a_,...)=(0,a_,a_,...)$ , the left shift $L(a_,a_,a_,...)=(a_,a_,a_,...)$ , and the projection map onto the first factor $P(a_,a_,a_,...)=(a_,0,0,...)$ . All three of these additive maps are not zero, and the composites $LP$ and $PR$ are both zero, so $L$ is a left zero divisor and $R$ is a right zero divisor in the ring of additive maps from $S$ to $S$ . However, $L$ is not a right zero divisor and $R$ is not a left zero divisor: the composite $LR$ is the identity. Note also that $RL$ is a two-sided zero-divisor since $RLP=0=PRL$ , while $LR=1$ is not in any direction.

Properties

• In the ring of n-by-n matrices over a field, the left and right zero divisors coincide; they are precisely the singular matrices. In the ring of n-by-n matrices over an integral domain, the zero divisors are precisely the matrices with determinant zero.
• Left or right zero divisors can never be units, because if a is invertible and ax = 0, then 0 = a−10 = a−1ax = x, whereas x must be nonzero.

Zero as a zero divisor

There is no need for a separate convention regarding the case a = 0, because the definition applies also in this case:

• If R is a ring other than the zero ring, then 0 is a (two-sided) zero divisor, because 0 · a = 0 = a · 0, where a is a nonzero element of R.
• If R is the zero ring, in which 0 = 1, then 0 is not a zero divisor, because there is no nonzero element that when multiplied by 0 yields 0.

Such properties are needed in order to make the following general statements true:

• In a nonzero commutative ring R, the set of non-zero-divisors is a multiplicative set in R. (This, in turn, is important for the definition of the total quotient ring.) The same is true of the set of non-left-zero-divisors and the set of non-right-zero-divisors in an arbitrary ring, commutative or not.
• In a commutative Noetherian ring R, the set of zero divisors is the union of the associated prime ideals of R.

Some references choose to exclude 0 as a zero divisor by convention, but then they must introduce exceptions in the two general statements just made.

Zero divisor on a module

Let R be a commutative ring, let M be an R-module, and let a be an element of R. One says that a is M-regular if the multiplication by a map $M{\stackrel {\to }}M$ is injective, and that a is a zero divisor on M otherwise. The set of M-regular elements is a multiplicative set in R.

Specializing the definitions of "M-regular" and "zero divisor on M" to the case M = R recovers the definitions of "regular" and "zero divisor" given earlier in this article.