Acta Cybernetica
Syntactic complexity of scattered context grammars
Acta Informatica
Journal of Automata, Languages and Combinatorics
Programmed Grammars and Classes of Formal Languages
Journal of the ACM (JACM)
Generative power of three-dimensional scattered context grammars
Theoretical Computer Science
Introduction To Automata Theory, Languages, And Computation
Introduction To Automata Theory, Languages, And Computation
Regulated Rewriting in Formal Language Theory
Regulated Rewriting in Formal Language Theory
An hierarchy between context-free and context-sensitive languages
Journal of Computer and System Sciences
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We show that, in the case of context-free programmed grammars with appearance checking working under free derivations, three nonterminals are enough to generate every recursively enumerable language. This improves the previously published bound of eight for the nonterminal complexity of these grammars. This also yields an improved nonterminal complexity bound of four for context-free matrix grammars with appearance checking. Moreover, we establish nonterminal complexity bounds for context-free programmed and matrix grammars working under leftmost derivations.