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Chomsky–Schützenberger representation theorem : ウィキペディア英語版
Chomsky–Schützenberger representation theorem
In formal language theory, the Chomsky–Schützenberger representation theorem is a theorem derived by Noam Chomsky and Marcel-Paul Schützenberger about representing a given context-free language in terms of two simpler languages. These two simpler languages, namely a regular language and a Dyck language, are combined by means of an intersection and a homomorphism.
A few notions from formal language theory are in order. A context-free language is ''regular'', if can be described by a regular expression, or, equivalently, if it is accepted by a finite automaton. A homomorphism is based on a function h which maps symbols from an alphabet \Gamma to words over another alphabet \Sigma; If the domain of this function is extended to words over \Gamma in the natural way, by letting h(xy)=h(x)h(y) for all words x and y, this yields a ''homomorphism'' h:\Gamma^
*\to \Sigma^
*. A ''matched alphabet'' T \cup \overline T is an alphabet with two equal-sized sets; it is convenient to think of it as a set of parentheses types, where T contains the opening parenthesis symbols, whereas the symbols in \overline T contains the closing parenthesis symbols. For a matched alphabet T \cup \overline T, the ''Dyck language'' D_T is given by
:D_T = \
words that are well-nested parentheses over T \cup \overline T.
:Chomsky–Schützenberger theorem. A language ''L'' over the alphabet \Sigma is context-free if and only if there exists
:
* a matched alphabet T \cup \overline T
:
* a regular language R over T \cup \overline T,
:
* and a homomorphism h : (T \cup \overline T)^
* \to \Sigma^
*
:such that L = h(D_T \cap R).
Proofs of this theorem are found in several textbooks, e.g. or .
==References==

*
*

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