In mathematics and computer science, the probabilistic automaton (PA) is a generalization of the nondeterministic finite automaton; it includes the probability of a given transition into the transition function, turning it into a transition matrix. Thus, the probabilistic automaton also generalizes the concepts of a Markov chain and of a subshift of finite type. The languages recognized by probabilistic automata are called stochastic languages; these include the regular languages as a subset. The number of stochastic languages is uncountable.
The concept was introduced by Michael O. Rabin in 1963; a certain special case is sometimes known as the Rabin automaton (not to be confused with the subclass of ?-automata also referred to as Rabin automata). In recent years, a variant has been formulated in terms of quantum probabilities, the quantum finite automaton.
The probabilistic automaton may be defined as an extension of a non-deterministic finite automaton , together with two probabilities: the probability of a particular state transition taking place, and with the initial state replaced by a stochastic vector giving the probability of the automaton being in a given initial state.
For the ordinary non-deterministic finite automaton, one has
Here, denotes the power set of .
so that if and if . The curried transition function can be understood to be a matrix with matrix entries
The matrix is then a square matrix, whose entries are zero or one, indicating whether a transition is allowed by the NFA. Such a transition matrix is always defined for a non-deterministic finite automaton.
The probabilistic automaton replaces these matrices by a family of right stochastic matrices , for each symbol a in the alphabet so that the probability of a transition is given by
A state change from some state to any state must occur with probability one, of course, and so one must have
for all input letters and internal states . The initial state of a probabilistic automaton is given by a row vector , whose components are the probabilities of the individual initial states , that add to 1:
The transition matrix acts on the right, so that the state of the probabilistic automaton, after consuming the input string , would be
In particular, the state of a probabilistic automaton is always a stochastic vector, since the product of any two stochastic matrices is a stochastic matrix, and the product of a stochastic vector and a stochastic matrix is again a stochastic vector. This vector is sometimes called the distribution of states, emphasizing that it is a discrete probability distribution.
Formally, the definition of a probabilistic automaton does not require the mechanics of the non-deterministic automaton, which may be dispensed with. Formally, a probabilistic automaton PA is defined as the tuple . A Rabin automaton is one for which the initial distribution is a coordinate vector; that is, has zero for all but one entries, and the remaining entry being one.
Let be the set of "accepting" or "final" states of the automaton. By abuse of notation, can also be understood to be the column vector that is the membership function for ; that is, it has a 1 at the places corresponding to elements in , and a zero otherwise. This vector may be contracted with the internal state probability, to form a scalar. The language recognized by a specific automaton is then defined as
A language is called ?-stochastic if and only if there exists some PA that recognizes the language, for fixed . A language is called stochastic if and only if there is some for which is ?-stochastic.
A cut-point is said to be an isolated cut-point if and only if there exists a such that
Every regular language is stochastic, and more strongly, every regular language is ?-stochastic. A weak converse is that every 0-stochastic language is regular; however, the general converse does not hold: there are stochastic languages that are not regular.
Every ?-stochastic language is stochastic, for some .
Every stochastic language is representable by a Rabin automaton.
If is an isolated cut-point, then is a regular language.
The p-adic languages provide an example of a stochastic language that is not regular, and also show that the number of stochastic languages is uncountable. A p-adic language is defined as the set of strings
in the letters .
That is, a p-adic language is merely the set of real numbers in [0, 1], written in base-p, such that they are greater than . It is straightforward to show that all p-adic languages are stochastic. In particular, this implies that the number of stochastic languages is uncountable. A p-adic language is regular if and only if is rational.
The probabilistic automaton has a geometric interpretation: the state vector can be understood to be a point that lives on the face of the standard simplex, opposite to the orthogonal corner. The transition matrices form a monoid, acting on the point. This may be generalized by having the point be from some general topological space, while the transition matrices are chosen from a collection of operators acting on the topological space, thus forming a semiautomaton. When the cut-point is suitably generalized, one has a topological automaton.
An example of such a generalization is the quantum finite automaton; here, the automaton state is represented by a point in complex projective space, while the transition matrices are a fixed set chosen from the unitary group. The cut-point is understood as a limit on the maximum value of the quantum angle.