A Generalization of Ogden's Lemma

Abstract

“Excluded positions” are incorporated into a modified form of Ogden's lemma, and a language satisfying the latter, which the authors' generalized lemma can show fails to be context-free, is presented. It is also shown that in a fairly general sense there ts no function relating the number of distinguished and excluded posnions which would allow the authors' Iteration lemma to characterize the context-free languages. © 1982, ACM. All rights reserved.