In mathematics and theoretical computer science, a semiautomaton is a deterministic finite automaton having inputs but no output. It consists of a set Q of states, a set Σ called the input alphabet, and a function T: Q × Σ → Q called the transition function.
Contents
Associated to any semiautomaton is a monoid called the characteristic monoid, input monoid, transition monoid or transition system of the semiautomaton, which acts on the set of states Q. This may be viewed either as an action of the free monoid of strings in the input alphabet Σ, or as the induced transformation semigroup of Q.
In older books like Clifford and Preston (1967) S-acts are called "operands".
In category theory, semiautomata essentially are functors.
A transformation semigroup or transformation monoid is a pair
Some authors regard "semigroup" and "monoid" as synonyms. Here a semigroup need not have an identity element; a monoid is a semigroup with an identity element (also called "unit"). Since the notion of functions acting on a set always includes the notion of an identity function, which when applied to the set does nothing, a transformation semigroup can be made into a monoid by adding the identity function.
M-acts
Let M be a monoid and Q be a non-empty set. If there exists a multiplicative operation
which satisfies the properties
for 1 the unit of the monoid, and
for all
The left act is defined similarly, with
and is often denoted as
An M-act is closely related to a transformation monoid. However the elements of M need not be functions per se, they are just elements of some monoid. Therefore, one must demand that the action of
Once one makes this demand, it is completely safe to drop all parenthesis, as the monoid product and the action of the monoid on the set are completely associative. In particular, this allows elements of the monoid to be represented as strings of letters, in the computer-science sense of the word "string". This abstraction then allows one to talk about string operations in general, and eventually leads to the concept of formal languages as being composed of strings of letters.
Another difference between an M-act and a transformation monoid is that for an M-act Q, two distinct elements of the monoid may determine the same transformation of Q. If we demand that this does not happen, then an M-act is essentially the same as a transformation monoid.
M-homomorphism
For two M-acts
for all
The M-acts and M-homomorphisms together form a category called M-Act.
Semiautomata
A semiautomaton is a triple
When the set of states Q is a finite set (it need not be!), a semiautomaton may be thought of as a deterministic finite automaton
Any semiautomaton induces an act of a monoid in the following way.
Let
For every word w in
Let
The set
Properties
If the set of states Q is finite, then the transition functions are commonly represented as state transition tables. The construction of all possible transitions driven by strings in the free group has a graphical depiction as de Bruijn graphs.
The set of states Q need not be finite, or even countable. As an example, semiautomata underpin the concept of quantum finite automata. There, the set of states Q are given by the complex projective space
The syntactic monoid of a formal language is isomorphic to the transition monoid of the minimal automaton accepting the language.