Examples of semigroups include: strings with the operation of concatenation (joining together); the set of n×n matrices together with the operation of multiplication; the set of transformations of a set and the operation of composing functions; the integers and the operation of choosing the maximum (or minimum) of two elements. The set of integers together with subtraction does not constitute a semigroup.
Semigroups play a major role in the theory of sequential machines and formal languages. If M is a sequential machine then any input string induces a function over the state-set of M. The set of all such induced functions forms a semigroup of the machine under function composition (see Myhill equivalence, Nerode equivalence). Semigroups are also used in certain aspects of computer arithmetic. See also free semigroup, transformation semigroup, monoid.
"semigroup." A Dictionary of Computing. . Encyclopedia.com. (August 20, 2018). http://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/semigroup
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