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In mathematics, a semigroup is an algebraic structure consisting of a set together with an associative binary operation.

The binary operation of a semigroup is most often denoted multiplicatively: x·y, or simply xy, denotes the result of applying the semigroup operation to the ordered pair (x, y). Associativity is formally expressed as that (x·yz = x·(y·z) for all x, y and z in the semigroup.

The name "semigroup" originates in the fact that a semigroup generalizes a group by preserving only associativity and closure under the binary operation from the axioms defining a group.[note 1] From the opposite point of view (of adding rather than removing axioms), a semigroup is an associative magma. As in the case of groups or magmas, the semigroup operation need not be commutative, so x·y is not necessarily equal to y·x; a typical example of associative but non-commutative operation is matrix multiplication. If the semigroup operation is commutative, then the semigroup is called a commutative semigroup or (less often than in the analogous case of groups) it may be called an abelian semigroup.

A monoid is an algebraic structure intermediate between groups and semigroups, 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 strings with concatenation as the binary operation, and the empty string as the identity element. Restricting to non-empty strings gives an example of a semigroup that is not a monoid. Positive integers with addition form a commutative semigroup that is not a monoid, whereas the non-negative integers 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 quasigroups, which are a generalization of groups in a different direction; the operation in a quasigroup need not be associative but quasigroups preserve from groups a notion of 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 a Cayley theorem for semigroups realizing any semigroup as transformation semigroup, in which arbitrary functions replace the role of bijections from group theory. Other fundamental techniques of studying semigroups like Green's relations do not imitate anything in group theory though. A deep result in the classification of finite semigroups is Krohn–Rhodes 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. In probability theory, semigroups are associated with Markov processes.[1][2] In other areas of applied mathematics, semigroups are fundamental models for linear time-invariant systems. 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 semigroups, orthodox semigroups, semigroups with involution, inverse semigroups and cancellative semigroups. There also interesting classes of semigroups that do not contain any groups except the trivial group; examples of the latter kind are bands and their commutative subclass—semilattices, which are also ordered algebraic structures.

Contents

DefinitionEdit

A semigroup is a set   together with a binary operation " " (that is, a function  ) that satisfies the associative property:

For all  , the equation   holds.

More succinctly, a semigroup is an associative magma.

Examples of semigroupsEdit

Basic conceptsEdit

Identity and zeroEdit

A left identity of a semigroup   (or more generally, magma) is an element   such that for all   in  ,  . Similarly, a right identity is an element   such that for all   in  ,  . Left and right identities are both called one-sided identities. A semigroup may have one or more left identities but no right identity, and vice versa.

A two-sided identity (or just identity) is an element that is both a left and right identity. Semigroups with a two-sided identity are called monoids. A semigroup may have at most one two-sided identity. If a semigroup has a two-sided identity, then the two-sided identity is the only one-sided identity in the semigroup. If a semigroup has both a left identity and a right identity, then it has a two-sided identity (which is therefore the unique one-sided identity).

A semigroup   without identity may be embedded in a monoid formed by adjoining an element   to   and defining   for all  .[3][4] The notation   denotes a monoid obtained from   by adjoining an identity if necessary (  for a monoid).[4]

Similarly, every magma has at most one absorbing element, which in semigroup theory is called a zero. Analogous to the above construction, for every semigroup  , one can define  , a semigroup with 0 that embeds  .

Subsemigroups and idealsEdit

The semigroup operation induces an operation on the collection of its subsets: given subsets A and B of a semigroup S, their product A · B, written commonly as AB, is the set { ab | a in A and b in B }. (This notion is defined identically as it is for groups.) In terms of this operation, a subset A is called

  • a subsemigroup if AA is a subset of A,
  • a right ideal if AS is a subset of A, and
  • a left ideal if SA is a subset of A.

If A is both a left ideal and a right ideal then it is called an ideal (or a two-sided ideal).

If S is a semigroup, then the intersection of any collection of subsemigroups of S is also a subsemigroup of S. So the subsemigroups of S form a complete lattice.

An example of semigroup with no minimal ideal is the set of positive integers under addition. The minimal ideal of a commutative semigroup, when it exists, is a group.

Green's relations, a set of five equivalence relations that characterise the elements in terms of the principal ideals they generate, are important tools for analysing the ideals of a semigroup and related notions of structure.

The subset with the property that its every element commutes with any other element of the semigroup is called the center of the semigroup.[5] The center of a semigroup is actually a subsemigroup.[6]

Homomorphisms and congruencesEdit

A semigroup homomorphism is a function that preserves semigroup structure. A function f: ST between two semigroups is a homomorphism if the equation

f(ab) = f(a)f(b).

holds for all elements a, b in S, i.e. the result is the same when performing the semigroup operation after or before applying the map f.

A semigroup homomorphism between monoids preserves identity if it is a monoid homomorphism. But there are semigroup homomorphisms which are not monoid homomorphisms, e.g. the canonical embedding of a semigroup   without identity into  . Conditions characterizing monoid homomorphisms are discussed further. Let   be a semigroup homomorphism. The image of   is also a semigroup. If   is a monoid with an identity element  , then   is the identity element in the image of  . If   is also a monoid with an identity element   and   belongs to the image of  , then  , i.e.   is a monoid homomorphism. Particularly, if   is surjective, then it is a monoid homomorphism.

Two semigroups S and T are said to be isomorphic if there is a bijection f : ST with the property that, for any elements a, b in S, f(ab) = f(a)f(b). Isomorphic semigroups have the same structure.

A semigroup congruence   is an equivalence relation that is compatible with the semigroup operation. That is, a subset   that is an equivalence relation and   and   implies   for every   in S. Like any equivalence relation, a semigroup congruence   induces congruence classes

 

and the semigroup operation induces a binary operation   on the congruence classes:

 

Because   is a congruence, the set of all congruence classes of   forms a semigroup with  , called the quotient semigroup or factor semigroup, and denoted  . The mapping   is a semigroup homomorphism, called the quotient map, canonical surjection or projection; if S is a monoid then quotient semigroup is a monoid with identity  . Conversely, the kernel of any semigroup homomorphism is a semigroup congruence. These results are nothing more than a particularization of the first isomorphism theorem in universal algebra. Congruence classes and factor monoids are the objects of study in string rewriting systems.

A nuclear congruence on S is one which is the kernel of an endomorphism of S.[7]

A semigroup S satisfies the maximal condition on congruences if any family of congruences on S, ordered by inclusion, has a maximal element. By Zorn's lemma, this is equivalent to saying that the ascending chain condition holds: there is no infinite strictly ascending chain of congruences on S.[8]

Every ideal I of a semigroup induces a subsemigroup, the Rees factor semigroup via the congruence x ρ y   ⇔   either x = y or both x and y are in I.

Structure of semigroupsEdit

For any subset A of S there is a smallest subsemigroup T of S which contains A, and we say that A generates T. A single element x of S generates the subsemigroup { xn | n is a positive integer }. If this is finite, then x is said to be of finite order, otherwise it is of infinite order. A semigroup is said to be periodic if all of its elements are of finite order. A semigroup generated by a single element is said to be monogenic (or cyclic). If a monogenic semigroup is infinite then it is isomorphic to the semigroup of positive integers with the operation of addition. If it is finite and nonempty, then it must contain at least one idempotent. It follows that every nonempty periodic semigroup has at least one idempotent.

A subsemigroup which is also a group is called a subgroup. There is a close relationship between the subgroups of a semigroup and its idempotents. Each subgroup contains exactly one idempotent, namely the identity element of the subgroup. For each idempotent e of the semigroup there is a unique maximal subgroup containing e. Each maximal subgroup arises in this way, so there is a one-to-one correspondence between idempotents and maximal subgroups. Here the term maximal subgroup differs from its standard use in group theory.

More can often be said when the order is finite. For example, every nonempty finite semigroup is periodic, and has a minimal ideal and at least one idempotent. The number of finite semigroups of a given size (greater than 1) is (obviously) larger than the number of groups of the same size. For example, of the sixteen possible "multiplication tables" for a set of two elements {a, b}, eight form semigroups[note 2] whereas only four of these are monoids and only two form groups. For more on the structure of finite semigroups, see Krohn–Rhodes theory.

Special classes of semigroupsEdit

  • A monoid is a semigroup with identity.
  • A subsemigroup is a subset of a semigroup that is closed under the semigroup operation.
  • A band is a semigroup the operation of which is idempotent.
  • A cancellative semigroup is one having the cancellation property:[9] a · b = a · c implies b = c and similarly for b · a = c · a.
  • A semilattice is a semigroup whose operation is idempotent and commutative.
  • 0-simple semigroups.
  • Transformation semigroups: any finite semigroup S can be represented by transformations of a (state-) set Q of at most |S| + 1 states. Each element x of S then maps Q into itself x: QQ and sequence xy is defined by q(xy) = (qx)y for each q in Q. Sequencing clearly is an associative operation, here equivalent to function composition. This representation is basic for any automaton or finite state machine (FSM).
  • The bicyclic semigroup is in fact a monoid, which can be described as the free semigroup on two generators p and q, under the relation pq = 1.
  • C0-semigroups.
  • Regular semigroups. Every element x has at least one inverse y satisfying xyx=x and yxy=y; the elements x and y are sometimes called "mutually inverse".
  • Inverse semigroups are regular semigroups where every element has exactly one inverse. Alternatively, a regular semigroup is inverse if and only if any two idempotents commute.
  • Affine semigroup: a semigroup that is isomorphic to a finitely-generated subsemigroup of Zd. These semigroups have applications to commutative algebra.

Structure theorem for commutative semigroupsEdit

There is a structure theorem for commutative semigroups in terms of semilattices.[10] A semilattice (or more precisely a meet-semilattice)   is a partially ordered set where every pair of elements   has a greatest lower bound, denoted  . The operation   makes   into a semigroup satisfying the additional idempotence law  .

Given a homomorphism   from an arbitrary semigroup to a semilattice, each inverse image   is a (possibly empty) semigroup. Moreover,   becomes graded by  , in the sense that

 

If   is onto, the semilattice   is isomorphic to the quotient of   by the equivalence relation   such that   iff  . This equivalence relation is a semigroup congruence, as defined above.

Whenever we take the quotient of a commutative semigroup by a congruence, we get another commutative semigroup. The structure theorem says that for any commutative semigroup  , there is a finest congruence   such that the quotient of   by this equivalence relation is a semilattice. Denoting this semilattice by  , we get a homomorphism   from   onto  . As mentioned,   becomes graded by this semilattice.

Furthermore, the components   are all Archimedean semigroups. An Archimedean semigroup is one where given any pair of elements  , there exists an element   and   such that  .

The Archimedean property follows immediately from the ordering in the semilattice  , since with this ordering we have   if and only if   for some   and  .

Group of fractionsEdit

The group of fractions or group completion of a semigroup S is the group G = G(S) generated by the elements of S as generators and all equations xy = z which hold true in S as relations.[11] There is an obvious semigroup homomorphism j : SG(S) which sends each element of S to the corresponding generator. This has a universal property for morphisms from S to a group:[12] given any group H and any semigroup homomorphism k : SH, there exists a unique group homomorphism f : GH with k=fj. We may think of G as the "most general" group that contains a homomorphic image of S.

An important question is to characterize those semigroups for which this map is an embedding. This need not always be the case: for example, take S to be the semigroup of subsets of some set X with set-theoretic intersection as the binary operation (this is an example of a semilattice). Since A.A = A holds for all elements of S, this must be true for all generators of G(S) as well: which is therefore the trivial group. It is clearly necessary for embeddability that S have the cancellation property. When S is commutative this condition is also sufficient[13] and the Grothendieck group of the semigroup provides a construction of the group of fractions. The problem for non-commutative semigroups can be traced to the first substantial paper on semigroups.[14][15] Anatoly Maltsev gave necessary and sufficient conditions for embeddability in 1937.[16]

Semigroup methods in partial differential equationsEdit

Semigroup theory can be used to study some problems in the field of partial differential equations. Roughly speaking, the semigroup approach is to regard a time-dependent partial differential equation as an ordinary differential equation on a function space. For example, consider the following initial/boundary value problem for the heat equation on the spatial interval (0, 1) ⊂ R and times t ≥ 0:

 

Let X = L2((0, 1) R) be the Lp space of square-integrable real-valued functions with domain the interval (0, 1) and let A be the second-derivative operator with domain

 

where H2 is a Hardy space. Then the above initial/boundary value problem can be interpreted as an initial value problem for an ordinary differential equation on the space X:

 

On an heuristic level, the solution to this problem "ought" to be u(t) = exp(tA)u0. However, for a rigorous treatment, a meaning must be given to the exponential of tA. As a function of t, exp(tA) is a semigroup of operators from X to itself, taking the initial state u0 at time t = 0 to the state u(t) = exp(tA)u0 at time t. The operator A is said to be the infinitesimal generator of the semigroup.

HistoryEdit

The study of semigroups trailed behind that of other algebraic structures with more complex axioms such as groups or rings. A number of sources[17][18] attribute the first use of the term (in French) to J.-A. de Séguier in Élements de la Théorie des Groupes Abstraits (Elements of the Theory of Abstract Groups) in 1904. The term is used in English in 1908 in Harold Hinton's Theory of Groups of Finite Order.

Anton Suschkewitsch obtained the first non-trivial results about semigroups. His 1928 paper "Über die endlichen Gruppen ohne das Gesetz der eindeutigen Umkehrbarkeit" ("On finite groups without the rule of unique invertibility") determined the structure of finite simple semigroups and showed that the minimal ideal (or Green's relations J-class) of a finite semigroup is simple.[18] From that point on, the foundations of semigroup theory were further laid by David Rees, James Alexander Green, Evgenii Sergeevich Lyapin, Alfred H. Clifford and Gordon Preston. The latter two published a two-volume monograph on semigroup theory in 1961 and 1967 respectively. In 1970, a new periodical called Semigroup Forum (currently edited by Springer Verlag) became one of the few mathematical journals devoted entirely to semigroup theory.

In recent years researchers in the field have become more specialized with dedicated monographs appearing on important classes of semigroups, like inverse semigroups, as well as monographs focusing on applications in algebraic automata theory, particularly for finite automata, and also in functional analysis.

GeneralizationsEdit

Group-like structures
Totalityα Associativity Identity Invertibility Commutativity
Semigroupoid Unneeded Required Unneeded Unneeded Unneeded
Category Unneeded Required Required Unneeded Unneeded
Groupoid Unneeded Required Required Required Unneeded
Magma Required Unneeded Unneeded Unneeded Unneeded
Quasigroup Required Unneeded Unneeded Required Unneeded
Loop Required Unneeded Required Required Unneeded
Semigroup Required Required Unneeded Unneeded Unneeded
Monoid Required Required Required Unneeded Unneeded
Group Required Required Required Required Unneeded
Abelian group Required Required Required Required Required
Closure, which is used in many sources, is an equivalent axiom to totality, though defined differently.

If the associativity axiom of a semigroup is dropped, the result is a magma, which is nothing more than a set M equipped with a binary operation M × MM.

Generalizing in a different direction, an n-ary semigroup (also n-semigroup, polyadic semigroup or multiary semigroup) is a generalization of a semigroup to a set G with a n-ary operation instead of a binary operation.[19] The associative law is generalized as follows: ternary associativity is (abc)de = a(bcd)e = ab(cde), i.e. the string abcde with any three adjacent elements bracketed. N-ary associativity is a string of length n + (n1) with any n adjacent elements bracketed. A 2-ary semigroup is just a semigroup. Further axioms lead to an n-ary group.

A third generalization is the semigroupoid, in which the requirement that the binary relation be total is lifted. As categories generalize monoids in the same way, a semigroupoid behaves much like a category but lacks identities.

Infinitary generalizations of commutative semigroups have sometimes been considered by various authors.[20]

See alsoEdit

NotesEdit

  1. ^ The closure axiom is implied by the definition of a binary operation on a set. Some authors thus omit it and specify three laws for a group and only one law (associativity) for semigroup.
  2. ^ Namely: the trivial semigroup in which (for all x and y) xy = a and its counterpart in which xy = b, the semigroups based on multiplication modulo 2 (choosing a or b as the identity element 1), the groups equivalent to addition modulo 2 (choosing a or b to be the identity element 0), and the semigroups in which the elements are either both left identities or both right identities.

CitationsEdit

  1. ^ (Feller 1971)
  2. ^ Martino, Ivan; Martino, Luca (2013-11-14). "On the variety of linear recurrences and numerical semigroups". Semigroup Forum. 88 (3): 569–574. doi:10.1007/s00233-013-9551-2. ISSN 0037-1912. 
  3. ^ Jacobson 2009, p. 30, ex. 5
  4. ^ a b Lawson 1998, p. 20
  5. ^ Kilp, Mati; Knauer, U.; Mikhalev, Aleksandr V. (2000). Monoids, Acts, and Categories: With Applications to Wreath Products and Graphs : a Handbook for Students and Researchers. Walter de Gruyter. p. 25. ISBN 978-3-11-015248-7. Zbl 0945.20036. 
  6. ^ Li͡apin, E. S. (1968). Semigroups. American Mathematical Soc. p. 96. ISBN 978-0-8218-8641-0. 
  7. ^ Lothaire 2011, p. 463
  8. ^ Lothaire 2011, p. 465
  9. ^ Clifford & Preston 1967, p. 3
  10. ^ Grillet 2001
  11. ^ Farb, B. (2006), Problems on mapping class groups and related topics, Amer. Math. Soc., p. 357, ISBN 0-8218-3838-5 
  12. ^ Auslander, M.; Buchsbaum, D. A. (1974). Groups, rings, modules. Harper & Row. p. 50. ISBN 0-06-040387-X. 
  13. ^ Clifford & Preston 1961, p. 34
  14. ^ (Suschkewitsch 1928)
  15. ^ Preston, G. B. (1990), Personal reminiscences of the early history of semigroups, retrieved 2009-05-12 
  16. ^ Maltsev, A. (1937), "On the immersion of an algebraic ring into a field", Math. Annalen, 113: 686–691, doi:10.1007/BF01571659. 
  17. ^ Earliest Known Uses of Some of the Words of Mathematics
  18. ^ a b An account of Suschkewitsch's paper by Christopher Hollings
  19. ^ Dudek, W.A. (2001), "On some old problems in n-ary groups", Quasigroups and Related Systems, 8: 15–36 
  20. ^ See references in Udo Hebisch and Hanns Joachim Weinert, Semirings and Semifields, in particular, Section 10, Semirings with infinite sums, in M. Hazewinkel, Handbook of Algebra, Vol. 1, Elsevier, 1996. Notice that in this context the authors use the term semimodule in place of semigroup.

ReferencesEdit

General references
Specific references