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 the 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]
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Null semigroup
Let S be a semigroup with zero element 0. Then S is called a null semigroup if for all x and y in S we have xy = 0.
Cayley table for a null semigroup
Let S = { 0, a, b, c } be a null semigroup. Then the Cayley table for S is as given below:
| 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 for all x and y in S we have xy = x.
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:
| 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 for all x and y in S we have xy = y.
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:
| a | b | c | |
|---|---|---|---|
| a | a | b | c |
| b | a | b | c |
| c | a | b | c |
References
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- ↑ M. Kilp, U. Knauer, A.V. Mikhalev, Monoids, Acts and Categories with Applications to Wreath Products and Graphs, De Gruyter Expositions in Mathematics vol. 29, Walter de Gruyter, 2000, ISBN 3-11-015248-7, p. 19
