Ogden's lemma for ET0L languages

Authors:
Max Rabkin
Affiliations:
School of Computer Science, University of the Witwatersrand, Johannesburg, South Africa
Venue:
LATA'12 Proceedings of the 6th international conference on Language and Automata Theory and Applications
Year:
2012

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Abstract

We develop a necessary condition for ET0L languages inspired by Ogden's Lemma. Besides being useful for proving that individual languages are not ET0L languages, this result also gives an alternative proof of Ehrenfeucht and Rozenberg's theorem about rare and nonfrequent symbols in ET0L languages.