On the impossibility of the homomorphic characterization of context sensitive languages
Theoretical Computer Science
On purely morphic characterizations of context-free languages
Theoretical Computer Science
Homomorphic characterizations of recursively enumerable languages with very small language classes
Theoretical Computer Science
Handbook of Formal Languages
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This paper concerns the Geffert normal forms for phrase structure grammars. We first generalize them to have a new formulation of minimal linear grammars with cancellation productions, called “cancel minimal linear grammars”. Then the generative powers of some classes of those grammars are investigated. It is shown that the class of languages generated by grammars with a unique {AB}-cancellation production properly includes the class of linear languages, while it is included in the class of context-free languages. Furthermore, the corresponding class of languages generated by grammars with a unique {AA}-cancellation production is shown to be a proper subclass of linear languages.