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. 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. According to A. H. Clifford and G. B. Preston, "In spite of their triviality, these semigroups arise naturally in a number of investigations."

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. 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. According to A. H. Clifford and G. B. Preston, "In spite of their triviality, these semigroups arise naturally in a number of investigations." ==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: ==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: ==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: ==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. On the other hand, a null (left/right zero) semigroup with an identity adjoined is called a find-unique (find-first/find-last) monoid. The class of null semigroups is: closed under taking subsemigroups closed under taking quotient of subsemigroup closed under arbitrary direct products. 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. ==See also== Right group ==References==

Text derived from https://en.wikipedia.org/w/index.php?oldid=1295010621 under CC BY-SA 4.0; modifications have been made.