tailieunhanh - Báo cáo khoa học: "Mildly Context-Sensitive Dependency Languages"

Dependency-based representations of natural language syntax require a fine balance between structural flexibility and computational complexity. In previous work, several constraints have been proposed to identify classes of dependency structures that are wellbalanced in this sense; the best-known but also most restrictive of these is projectivity. Most constraints are formulated on fully specified structures, which makes them hard to integrate into models where structures are composed from lexical information. . | Mildly Context-Sensitive Dependency Languages Marco Kuhlmann Programming Systems Lab Saarland University Saarbrucken Germany kuhlmann@ Mathias Mohl Programming Systems Lab Saarland University Saarbrucken Germany mmohl@ Abstract Dependency-based representations of natural language syntax require a fine balance between structural flexibility and computational complexity. In previous work several constraints have been proposed to identify classes of dependency structures that are well-balanced in this sense the best-known but also most restrictive of these is projectivity. Most constraints are formulated on fully specified structures which makes them hard to integrate into models where structures are composed from lexical information. In this paper we show how two empirically relevant relaxations of projectivity can be lexicalized and how combining the resulting lexicons with a regular means of syntactic composition gives rise to a hierarchy of mildly context-sensitive dependency languages. 1 Introduction Syntactic representations based on word-to-word dependencies have a long tradition in descriptive linguistics. Lately they have also been used in many computational tasks such as relation extraction Cu-lotta and Sorensen 2004 parsing McDonald et al. 2005 and machine translation Quirk et al. 2005 . Especially in recent work on parsing there is a particular interest in non-projective dependency structures in which a word and its dependents may be spread out over a discontinuous region of the sentence. These structures naturally arise in the syntactic analysis of languages with flexible word order such 160 as Czech Veselá et al. 2004 . Unfortunately most formal results on non-projectivity are discouraging While grammar-driven dependency parsers that are restricted to projective structures can be as efficient as parsers for lexicalized context-free grammar Eisner and Satta 1999 parsing is prohibitively expensive when unrestricted forms of .