Lexical Information for Determining Japanese Unbounded Dependency

Lexical Information for Determining Japanese Unbounded Dependency Shin-ichiro KAMEI, Kazunori MURAKI and Shin'ichi DOI Information Teclmology Research Laboratories, NEC Corporation 4-1-1 Miyazaki, Miyamae-ku, Kawasaki 216, JAPAN {kamei, k-muraki, doi}@hum, cl. nec. co. jp Abstract This paper presents a practical method for a global structure analyzing Mgorithm of Japanese long sen- tences with lexical information, a method which we call Lexical Discourse Grammar (LDG). This method assumes that .Japanese function words, such as con- junctive particles (postpositions) located at the end of each clause, have modality and suggest global struc- tures of Japanese long sentences in cooperation with modality within predicates or auxiliary verbs. LDG classifies the encapsulating powers of function words into six levels, and modality in predicates into four types. LDG presumes tile inter-clausal dependency within Japanese sentences prior to syntactic and se- mantic analyses, by utilizing the differences of the en- capsulating powers each Japanese function word has, and by utilizing modification preference between func- tion words and predicates that reflects consistency of modality in them. In order to confirm the encapsula- tion power of Japanese function words, we analyzed the speech utterances of a male announcer and found the correlation between a particle's encapsulating power and the pause length inserted after the clause with a conjunctive particle. 1 Introduction When analyzing long sentences with two or more pred- icates (i.e. compound and complex sentences), it is dif- ficult to grasp the proper structure of sentences hav- ing a large nmnber of possible dependency (modifier- modifee relation) structures. This difficulty is more marked in Japanese than in English, since there are more syntactically ambiguous structures in Japanese. Tile Japanese language has few syntactic indicators for dividing sentences into phrases or clauses, unlike En- glish with its relative pronouns and subordinate con- junctions. One of the most critical features of Japanese is that the difference between a phra~se and a clause is not cleat'. Even subjects or other obligatory elements of clauses are omitted very often when they aye indi- cated by contexts. In addition, the Japanese language does not have any parts of speech to clearly indicate either the beginning or end of a phrase or a clause. Another critical feature is that the Japanese language is an almost pure Head-final language, i.e., predicates and function words to signify the sentence structure ap- pear at the end of the clause or sentence. This means that it is syntactically possible for all phrases or clauses that can modit) predicates to modify all other phrases or clauses that appear in the latter part of long sen- tences. These syntactic characteristics of the Japanese lan- guage make it difficult to determine the dependency (modification) structure of hmg sentences. Simple parsing of Japanese long sentences inevitably produces a huge number of possible modification structures. A conventional bottom-up parsing method can reduce ambiguity in modification by local information in tim surface structure. However, this inclines toward an improper output, since the locally highest likelihood is sometimes low on the whole. To overcome this …

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