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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 , 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 * a finite set of states * a finite set of input symbols * a transition function * a set of states distinguished as ''accepting'' (or ''final'') ''states'' . Here, denotes the power set of . By use of currying, the transition function of a non-deterministic finite automaton can be written as a membership function : 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 this matrix by a stochastic matrix , 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 add to unity: : 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「probabilistic automaton」の詳細全文を読む スポンサード リンク
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