Semigroup action

In algebra and theoretical computer science, an action or act of a semigroup on a set is a rule which associates to each element of the semigroup a transformation of the set in such a way that the product of two elements of the semigroup (using the semigroup operation) is associated with the composite of the two corresponding transformations. The terminology conveys the idea that the elements of the semigroup are "acting" as transformations of the set. From an algebraic perspective, a semigroup action is a generalization of the notion of a group action in group theory. From the computer science point of view, semigroup actions are closely related to automata: the set models the state of the automaton and the action models transformations of that state in response to inputs.
An important special case is a monoid action or act, in which the semigroup is a monoid and the identity element of the monoid acts as the identity transformation of a set. From a category theoretic point of view, a monoid is a category with one object, and an act is a functor from that category to the category of sets. This immediately provides a generalization to monoid acts on objects in categories other than the category of sets.
Another important special case is a transformation semigroup. This is a semigroup of transformations of a set, and hence it has a tautological action on that set. This concept is linked to the more general notion of a semigroup by an analogue of Cayley's theorem.
"(A note on terminology: the terminology used in this area varies, sometimes significantly, from one author to another. See the article for details.)"
Formal definitions.
Let "S" be a semigroup. Then a (left) semigroup action (or act) of "S" is a set "X" together with an operation which is compatible with the semigroup operation ∗ as follows:
This is the analogue in semigroup theory of a (left) group action, and is equivalent to a semigroup homomorphism into the set of functions on "X". Right semigroup actions are defined in a similar way using an operation satisfying .
If "M" is a monoid, then a (left) monoid action (or act) of "M" is a (left) semigroup action of "M" with the additional property that
where "e" is the identity element of "M". This correspondingly gives a monoid homomorphism. Right monoid actions are defined in a similar way. A monoid "M" with an action on a set is also called an operator monoid.
A semigroup action of "S" on "X" can be made into monoid act by adjoining an identity to the semigroup and requiring that it acts as the identity transformation on "X".
Terminology and notation.
If "S" is a semigroup or monoid, then a set "X" on which "S" acts as above (on the left, say) is also known as a (left) S"-act, S"-set, S"-action, S"-operand, or left act over "S". Some authors do not distinguish between semigroup and monoid actions, by regarding the identity axiom () as empty when there is no identity element, or by using the term unitary "S"-act for an "S"-act with an identity.
The defining property of an act is analogous to the associativity of the semigroup operation, and means that all parentheses can be omitted. It is common practice, especially in computer science, to omit the operations as well so that both the semigroup operation and the action are indicated by juxtaposition. In this way strings of letters from "S" act on "X", as in the expression "stx" for "s", "t" in "S" and "x" in "X".
It is also quite common to work with right acts rather than left acts. However, every right S-act can be interpreted as a left act over the opposite semigroup, which has the same elements as S, but where multiplication is defined by reversing the factors, , so the two notions are essentially equivalent. Here we primarily adopt the point of view of left acts.
Acts and transformations.
It is often convenient (for instance if there is more than one act under consideration) to use a letter, such as formula_1, to denote the function
defining the formula_3-action and hence write formula_4 in place of formula_5. Then for any formula_6 in formula_3, we denote by
the transformation of formula_9 defined by
By the defining property of an formula_3-act, formula_1 satisfies
Further, consider a function formula_14. It is the same as formula_15 (see "Currying"). Because formula_16 is a bijection, semigroup actions can be defined as functions formula_17 which satisfies
That is, formula_1 is a semigroup action of formula_3 on formula_9 if and only if formula_22 is a semigroup homomorphism from formula_3 to the full transformation monoid of formula_9.
"S"-homomorphisms.
Let "X" and "X"′ be "S"-acts. Then an "S"-homomorphism from "X" to "X"′ is a map
such that 
The set of all such "S"-homomorphisms is commonly written as formula_29.
"M"-homomorphisms of "M"-acts, for "M" a monoid, are defined in exactly the same way.
"S"-Act and "M"-Act.
For a fixed semigroup "S", the left "S"-acts are the objects of a category, denoted "S"-Act, whose morphisms are the "S"-homomorphisms. The corresponding category of right "S"-acts is sometimes denoted by Act-"S". (This is analogous to the categories "R"-Mod and Mod-"R" of left and right modules over a ring.)
For a monoid "M", the categories "M"-Act and Act-"M" are defined in the same way.
Transformation semigroups.
A correspondence between transformation semigroups and semigroup actions is described below. If we restrict it to faithful semigroup actions, it has nice properties.
Any transformation semigroup can be turned into a semigroup action by the following construction. For any transformation semigroup formula_3 of formula_9, define a semigroup action formula_1 of formula_3 on formula_9 as formula_55 for formula_56. This action is faithful, which is equivalent to formula_57 being injective.
Conversely, for any semigroup action formula_1 of formula_3 on formula_9, define a transformation semigroup formula_61. In this construction we "forget" the set formula_3. formula_63 is equal to the image of formula_57. Let us denote formula_57 as formula_66 for brevity. If formula_66 is injective, then it is a semigroup isomorphism from formula_3 to formula_63. In other words, if formula_1 is faithful, then we forget nothing important. This claim is made precise by the following observation: if we turn formula_63 back into a semigroup action formula_72 of formula_63 on formula_9, then formula_75 for all formula_76. formula_1 and formula_72 are "isomorphic" via formula_66, i.e., we essentially recovered formula_1. Thus some authors see no distinction between faithful semigroup actions and transformation semigroups.
Applications to computer science.
Semiautomata.
Transformation semigroups are of essential importance for the structure theory of finite state machines in automata theory. In particular, a "semiautomaton" is a triple ("Σ","X","T"), where "Σ" is a non-empty set called the "input alphabet", "X" is a non-empty set called the "set of states" and "T" is a function
called the "transition function". Semiautomata arise from deterministic automata by ignoring the initial state and the set of accept states.
Given a semiautomaton, let "T""a": "X" → "X", for "a" ∈ "Σ", denote the transformation of "X" defined by "T""a"("x") = "T"("a","x"). Then the semigroup of transformations of "X" generated by {"T""a" : "a" ∈ "Σ"} is called the "characteristic semigroup" or "transition system" of ("Σ","X","T"). This semigroup is a monoid, so this monoid is called the "characteristic" or "transition monoid". It is also sometimes viewed as a "Σ"∗-act on "X", where "Σ"∗ is the free monoid of strings generated by the alphabet "Σ", and the action of strings extends the action of "Σ" via the property
Krohn–Rhodes theory.
Krohn–Rhodes theory, sometimes also called "algebraic automata theory", gives powerful decomposition results for finite transformation semigroups by cascading simpler components.