A variant of random context grammars: semi-conditional grammars
Theoretical Computer Science
A shrinking lemma for random forbidding context languages
Theoretical Computer Science
A pumping lemma for random permitting context languages
Theoretical Computer Science
Regulated Rewriting in Formal Language Theory
Regulated Rewriting in Formal Language Theory
Grammars with Context Conditions and Their Applications
Grammars with Context Conditions and Their Applications
Introduction to Automata Theory, Languages, and Computation (3rd Edition)
Introduction to Automata Theory, Languages, and Computation (3rd Edition)
LATA '09 Proceedings of the 3rd International Conference on Language and Automata Theory and Applications
Simple restriction in context-free rewriting
Journal of Computer and System Sciences
On erasing productions in random context grammars
ICALP'10 Proceedings of the 37th international colloquium conference on Automata, languages and programming: Part II
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Context-free grammars are widely used for the simple form of their rules. A derivation step consists of the choice of a nonterminal of the sentential form and of an application of a rule rewriting it. Several regulations of the derivation process have been studied to increase the power of context-free grammars. In the resulting grammars, however, not only the symbols to be rewritten are restricted, but also the rules to be applied. In this paper, we study context-free grammars with a simpler restriction where only symbols to be rewritten are restricted, not the rules, in the sense that any rule rewriting the chosen nonterminal can be applied. We prove that these grammars have the same power as random context, matrix, or programmed grammars. We also present two improved normal forms and discuss the characterization of context-sensitive languages by a variant using strings of length at most two instead of symbols.