Lexical Information for Determining Japanese Unbounded Dependency

This paper presents a practical method for a global structure analyzing Mgorithm of Japanese long sentences with lexical information, a method which we call Lexical Discourse Grammar (LDG). This method assumes that .Japanese function words, such as conjunctive particles (postpositions) located at the end of each clause, have modality and suggest global structures 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 semantic analyses, by utilizing the differences of the encapsulating powers each Japanese function word has, and by utilizing modification preference between function words and predicates that reflects consistency of modality in them. In order to confirm the encapsulation 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 I n t r o d u c t i o n When analyzing long sentences with two or more predicates (i.e. compound and complex sentences), it is difficult to grasp the proper structure of sentences having a large nmnber of possible dependency (modifiermodifee 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 English with its relative pronouns and subordinate conjunctions. 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 indicated 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 appear 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 sentences. These syntactic characteristics of the Japanese language 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 problem, several methods to predict the global structure of long sentences have been proposed. One is a top-down parsing method by matching the input sentence and the domain-specific patterns (Furuse et al., 1992). Improvements made by other researchers enabled this method to parse irregular, incomplete and multiplex patterns, by describing the domain-dependent patterns in the form of grammar (Doi, Muraki, et M., 1993). Another method employs global structure presumption to divide a sentence into clauses by utilizing general lexical information. It predicts the sentence structure prior to syntactic analysis only by utilizing domain-independent lexical information such as conjunctive particles, parallel expressions, theme transition, etc. (Mizuno et al., 1990; Kurohashi et al., 1992). Lexical Discourse Grammar (LDG) is one of the approaches with which a global structure of a long sentence is presumed by focusing on function words (Kamei et al, 1986; Doi et al., 1991). LDG assumes that Japanese function words, such as conjunctive partides (postpositions) located at the end of each clause, convey modMity, or propositional attitude, and suggest global structures of Japanese long sentences in cooperation with modality in predicates, especially within