Editing Semigroup (section)

{{Short description|Algebraic structure consisting of a set with an associative binary operation}}
<!-- Commenting out the following image file that may be replaced by something more appropriate, as this image does not properly clarify what associative property (of string concatenation or anything else) is and creates confusion rather than explains anything:
[[File:Semigroup associative image.svg|thumb|right|upright=1.75|The associative property of string concatenation.]] -->
[[File:Magma to group4.svg|thumb|right|300px|Algebraic structures between [[Magma (algebra)|magmas]] and [[Group (mathematics)|groups]]: A ''semigroup'' is a [[magma (algebra)|magma]] with [[Associative property|associativity]]. A [[monoid]] is a ''semigroup'' with an [[identity element]].]]
In mathematics, a '''semigroup''' is an [[algebraic structure]] consisting of a [[Set (mathematics)|set]] together with an [[associative]] internal [[binary operation]] on it.

The binary operation of a semigroup is most often denoted multiplicatively (just notation, not necessarily the elementary arithmetic [[multiplication]]): {{nowrap|''x'' ⋅ ''y''}}, or simply ''xy'', denotes the result of applying the semigroup operation to the [[ordered pair]] {{nowrap|(''x'', ''y'')}}. Associativity is formally expressed as that {{nowrap|1=(''x'' ⋅ ''y'') ⋅ ''z'' = ''x'' ⋅ (''y'' ⋅ ''z'')}} for all ''x'', ''y'' and ''z'' in the semigroup.

Semigroups may be considered a special case of [[magma (algebra)|magmas]], where the operation is associative, or as a generalization of [[group (mathematics)|groups]], without requiring the existence of an identity element or inverses.{{efn|The closure axiom is implied by the definition of a binary operation on a set. Some authors thus omit it and specify three axioms for a group and only one axiom (associativity) for a semigroup.}} As in the case of groups or magmas, the semigroup operation need not be [[commutativity|commutative]], so {{nowrap|''x'' ⋅ ''y''}} is not necessarily equal to {{nowrap|''y'' ⋅ ''x''}}; a well-known example of an operation that is associative but non-commutative is [[matrix multiplication]]. If the semigroup operation is commutative, then the semigroup is called a ''commutative semigroup'' or (less often than in the [[abelian group|analogous case of groups]]) it may be called an ''abelian semigroup''.

A [[monoid]] is an algebraic structure intermediate between semigroups and groups, and is a semigroup having an [[identity element]], thus obeying all but one of the axioms of a group: existence of inverses is not required of a monoid. A natural example is [[string (computer science)|strings]] with [[concatenation]] as the binary operation, and the empty string as the identity element. Restricting to non-empty [[string (computer science)|strings]] gives an example of a semigroup that is not a monoid. Positive [[integer]]s with addition form a commutative semigroup that is not a monoid, whereas the non-negative [[integer]]s do form a monoid. A semigroup without an identity element can be easily turned into a monoid by just adding an identity element. Consequently, monoids are studied in the theory of semigroups rather than in group theory. Semigroups should not be confused with [[quasigroup]]s, which are generalization of groups in a different direction; the operation in a quasigroup need not be associative but quasigroups [[Group (mathematics)#Division|preserve from groups]] the notion of [[Division (mathematics)|division]]. Division in semigroups (or in monoids) is not possible in general.

The formal study of semigroups began in the early 20th century. Early results include [[Transformation semigroup#Cayley representation|a Cayley theorem for semigroups]] realizing any semigroup as a [[transformation semigroup]], in which arbitrary functions replace the role of bijections in group theory. A deep result in the classification of finite semigroups is [[Krohn–Rhodes theory]], analogous to the [[Jordan–Hölder decomposition]] for finite groups. Some other techniques for studying semigroups, like [[Green's relations]], do not resemble anything in group theory.

The theory of finite semigroups has been of particular importance in [[theoretical computer science]] since the 1950s because of the natural link between finite semigroups and [[finite automata]] via the [[syntactic monoid]]<!-- {{sfn|ps=|Eilenberg|1973}} -->.  In [[probability theory]], semigroups are associated with [[Markov process]]es.{{sfn|ps=|Feller|1971}} In other areas of [[applied mathematics]], semigroups are fundamental models for [[linear time-invariant system]]s.  In [[partial differential equations]], a semigroup is associated to any equation whose spatial evolution is independent of time.

There are numerous [[special classes of semigroups]], semigroups with additional properties, which appear in particular applications. Some of these classes are even closer to groups by exhibiting some additional but not all properties of a group. Of these we mention: [[regular semigroup]]s, [[orthodox semigroup]]s, [[semigroup with involution|semigroups with involution]], [[inverse semigroup]]s and [[cancellative semigroup]]s. There are also interesting classes of semigroups that do not contain any groups except the [[trivial group]]; examples of the latter kind are [[band (mathematics)|bands]] and their commutative subclass – [[semilattice]]s, which are also [[:Category:Ordered algebraic structures|ordered algebraic structure]]s.

{{Algebraic structures |Group}}