# Null semigroup

In mathematics, a null semigroup (also called a zero semigroup) is a semigroup with an absorbing element, called zero, in which the product of any two elements is zero.[1] If every element of a semigroup is a left zero then the semigroup is called a left zero semigroup; a right zero semigroup is defined analogously.[2] According to Clifford and Preston, "In spite of their triviality, these semigroups arise naturally in a number of investigations."[1]

## Null semigroup

Let S be a semigroup with zero element 0. Then S is called a null semigroup if xy = 0 for all x and y in S.

### Cayley table for a null semigroup

Let S = {0, a, b, c} be (the underlying set of) a null semigroup. Then the Cayley table for S is as given below:

Cayley table for a null semigroup
0 a b c
0 0 0 0 0
a 0 0 0 0
b 0 0 0 0
c 0 0 0 0

## Left zero semigroup

A semigroup in which every element is a left zero element is called a left zero semigroup. Thus a semigroup S is a left zero semigroup if xy = x for all x and y in S.

### Cayley table for a left zero semigroup

Let S = {a, b, c} be a left zero semigroup. Then the Cayley table for S is as given below:

Cayley table for a left zero semigroup
a b c
a a a a
b b b b
c c c c

## Right zero semigroup

A semigroup in which every element is a right zero element is called a right zero semigroup. Thus a semigroup S is a right zero semigroup if xy = y for all x and y in S.

### Cayley table for a right zero semigroup

Let S = {a, b, c} be a right zero semigroup. Then the Cayley table for S is as given below:

Cayley table for a right zero semigroup
a b c
a a b c
b a b c
c a b c

## Properties

A non-trivial null (left/right zero) semigroup does not contain an identity element. It follows that the only null (left/right zero) monoid is the trivial monoid.

The class of null semigroups is:

It follows that the class of null (left/right zero) semigroups is a variety of universal algebra, and thus a variety of finite semigroups. The variety of finite null semigroups is defined by the identity ab = cd.