New Frontiers Beyond Context-Freeness: DI-Grammars And DI-Automata

A new class of formal languages will be defined the Distributed Index Languages (DI-languages). The grammar-formalism generating the new class - the DI-grammars - cover unbound dependencies in a rather natural way. The place of DI-languages in the Chomsky-hierarchy will be determined: Like Aho's indexed Languages, DI-languages represent a proper subclass of Type 1 (contextsensitive languages) and properly include Type 2 (context-free languages), but the DI-class is neither a subclass nor a superclass of Aho's indexed class. It will be shown that, apart from DI-grammars, DI-languages can equivalently be characterized by a special type of automata - DI-automata. Finally, the time complexity of the recognition-problem for an interesting subclass of DI-Grammars will approximately be determined.

• Type: Conference
• Year: 1993
• Where: EACL
• Authors: Peter Staudacher
• Keywords: Aho's Indexed Languages, Cover Unbound Dependencies, EACL 1993, Formal Languages, Natural Language Processing