Introduction to Formal Language Theory
Introduction to Formal Language Theory
Automata: Theoretic Aspects of Formal Power Series
Automata: Theoretic Aspects of Formal Power Series
MFCS '00 Proceedings of the 25th International Symposium on Mathematical Foundations of Computer Science
An efficient context-free parsing algorithm
An efficient context-free parsing algorithm
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We prove that the class of context-free languages with polynomially bounded ambiguity (PCFL) is the closure of the class of unambiguous languages (UCFL) under projections which deletes Parikh bounded symbols only. A symbol a is Parikh bounded in a language L if there is a constant c such that no word of L contains more than c occurrences of a. Furthermore PCFL is closed under the formally stronger operation of Parikh bounded substitution, i.e., a substitution which is the identity for non Parikh bounded symbols. Finally we prove that the closure of UCFL under union and concatenation is a proper subset of PCFL.