tailieunhanh - Báo cáo khoa học: "STRUCTURAL DISAMBIGUATION WITH CONSTRAINT PROPAGATION"

We present a new grammatical formalism called Constraint Dependency G r a m m a r (CDG) in which every grammatical rule is given as a constraint on wordto-word modifications. CDG parsing is formalized as a constraint satisfaction problem over a finite domain so that efficient constraint-propagation algorithms can be employed to reduce structural ambiguity without generating individual parse trees. The weak generative capacity and the computational complexity of CDG parsing are also discussed. | STRUCTURAL DISAMBIGUATION WITH CONSTRAINT PROPAGATION Hừoshi Maruyama IBM Research Tokyo Research Laboratory 5-19 Sanbancho Chiyoda-ku Tokyo 102 Japan Abstract We present a new grammatical formalism called Constraint Dependency Grammar CDG in which every grammatical rule is given as a constraint on word-to-word modifications. CDG parsing is formalized as a constraint satisfaction problem over a finite domain so that efficient constraint-propagation algorithms can be employed to reduce structural ambiguity without generating individual parse trees. The weak generative capacity and the computational complexity of CDG parsing are also discussed. 1 INTRODUCTION We are interested in an efficient treatment of structural ambiguity in natural language analysis. It is known that every-way ambiguous constructs such as prepositional attachment in English have a Catalan number of ambiguous parses Church and Patil 1982 which grows at a faster than exponential rate Knuth 1975 . A parser should be provided with a disambiguation mechanism that does not involve generating such a combinatorial number of parse trees explicitly. We have developed a parsing method in which an intermediate parsing result is represented as a data structure called a constraint network. Every solution that satisfies all the constraints simultaneously corresponds to an individual parse tree. No explicit parse trees are generated until ultimately necessary. Parsing and successive disambiguation are performed by adding new constraints to the constraint network. Newly added constraints are efficiently propagated over the network by Constraint Propagation Waltz 1975 Montanari 1976 to remove inconsistent values. In this paper we present the basic ideas of a formal grammatical theory called Constraint Dependency Grammar CDG for short that makes this parsing technique possible. CDG has a reasonable time bound in its parsing while its weak generative capacity is strictly greater than that .