Symmetric Pattern Matching Analysis for English Coordinate Structures

Symmetric Pattern Matching Analysis for English Coordinate Structures Akitoshi Okumura and Kazunori Muraki NEC Corp. Information Technology Research Laboratories 4-1-1 Miyazaki, Miyamae-ku, Kawasaki, Kanagawa 216, JAPAN okumura@hum.cl.nec.co.jp Abstract The authors propose a model for analyz- ing English sentences including coordinate conjunctions such as "and", "or", "but" and the equivalent words. Syntactic analysis of the English coordinate sentences is one of the most difficult problems for machine translation (MT) systems. The problem is selecting, from all possible candidates, the correct syntactic structure formed by an individual coordinate conjunction, i.e. determining which constituents are coor- dinated by the conjunction. Typically, so many possible structures are produced that MT systems cannot select the correct one, even if the grammars allow to write the rules in the simple notations. This pa- per presents an English coordinate struc- ture analysis model, which provides top- down scope information of the correct syn- tactic structure by taking advantage of the symmetric patterns of the parallelism. The model is based on a balance matching oper- ation for two lists of the feature sets, which provides four effects: the reduction of anal- ysis cost, the improvement of word dis- ambiguation, the interpretation of ellipses, and robust analysis. This model was prac- tically implemented and incorporated into the English-Japanese MT system, and pro- vided about 75% accuracy in the practical translation use. 1 Introduction This paper presents a model for analyzing English sentences including coordinate conjunctions such as "and" , "or" , "but" and equivalent words. Syntactic analysis of English coordinate sentences is one of the most difficult problems for machine translation (MT) systems. The problem is select- ing, from all possible candidates, the correct syn- tactic structure formed by an individual coordinate conjunction, i.e., determining which constituents are conjoined by the conjunction. Although the con- junction appears to have a simple function in the English language, it has been researched as a con- junct scope problem by both theoretical and com- putational linguists. Theoretically, it is possible to describe the syntactic and semantic constraints that govern the acceptability of a structure in which two constituents are conjoined by the conjunction(Lesmo and Torasso, 1985; Gazdar, 1981; Schachter, 1977). Computationally, it is possible to describe the gram- mar and heuristic rules for these constraints by ATN networks, logic grarnmars~ HPSG~ and cate- gorical grammars(Kosy, 1986; Fong and Berwick, 1985; Huang, 1984; Boguraev et al., 1983; Blackwell, 1981; Niimi et al., 1986). However, it is not easy to apply these techniques to large-scale MT systems, because there exist a variety of conjoined patterns, many word ambiguities, some unknown words and ellipses of the words simultaneously, in real environ- ments. Also, there may be several conjunctions and the equivalent words, such as commas, in a single sentence. Typically, the methods produce so many possible structures that MT systems cannot select the correct one, even if the grammars allow to write the rules in the simple notations. Often, conjunctions might produce the reading difficulty even for the human readers. However, they also give the …

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