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probabilistic automaton : ウィキペディア英語版
probabilistic automaton
In mathematics and computer science, the probabilistic automaton (PA) is a generalization of the non-deterministic finite automaton; it includes the probability of a given transition into the transition function, turning it into a transition matrix or stochastic matrix. Thus, the probabilistic automaton generalizes the concept of a Markov chain or 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. In recent years, a variant has been formulated in terms of quantum probabilities, the quantum finite automaton.
==Definition==
The probabilistic automaton may be defined as an extension of a non-deterministic finite automaton (Q,\Sigma,\delta,q_0,F), together with two probabilities: the probability P of a particular state transition taking place, and with the initial state q_0 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
* a finite set of states Q
* a finite set of input symbols \Sigma
* a transition function \delta:Q\times\Sigma \to P(Q)
* a set of states F distinguished as ''accepting'' (or ''final'') ''states'' F\subset Q.
Here, P(Q) denotes the power set of Q.
By use of currying, the transition function \delta:Q\times\Sigma \to P(Q) of a non-deterministic finite automaton can be written as a membership function
:\delta:Q\times\Sigma \times Q\to \
so that \delta(q,a,q^\prime)=1 if q^\prime\in \delta(q,a) and \delta(q,a,q^\prime)=0 if q^\prime\notin \delta(q,a). The curried transition function can be understood to be a matrix with matrix entries
:\left()_=\delta(q,a,q^\prime)
The matrix \theta_a is then a square matrix, whose entries are zero or one, indicating whether a transition q\stackrel q^\prime is allowed by the NFA. Such a transition matrix is always defined for a non-deterministic finite automaton.
The probabilistic automaton replaces this matrix by a stochastic matrix P, so that the probability of a transition is given by
:\left()_
A state change from some state to any state must occur with probability one, of course, and so one must have
:\sum_\left()_=1
for all input letters a and internal states q. The initial state of a probabilistic automaton is given by a row vector v, whose components add to unity:
:\sum_\left()_=1
The transition matrix acts on the right, so that the state of the probabilistic automaton, after consuming the input string abc, would be
:v P_a P_b P_c
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 (Q,\Sigma,P, v, F). A Rabin automaton is one for which the initial distribution v is a coordinate vector; that is, has zero for all but one entries, and the remaining entry being one.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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