An Efficient Treatment Of Japanese Verb Inflection For Morphological Analysis

Because of its simple appearance, Japanese verb inflection has never been treated seriously. In this paper we reconsider traditional lexical treatments of Japanese verb inflection, and propose a new treatment of verb inflection which uses newlydevised segmenting units. We show that our proposed treatment minimizes the number of lexical entries and avoids useless segmentation. It requires 20 to 40% less chart parsing computation and it is also suitable for error correction in optical character readers. Introduct ion In this paper we focus on lexical entries for coping with Japanese verb inflection. The problem of treating verb inflection comes from the nature of written Japanese, in which word boundaries are not usually indicated explicitly. The morphological analyzer must therefore check for the existence of a verb and its inflection at each position in an input character string. As a consequence, an awkward treatment of verb inflection may result unacceptably low computational efficiency. Japanese verb inflection seems to be quite simple. Therefore, it has never been a central subject of natural language processing (NLP) studies. It is also because, in the early stages of Japanese NLP, the most time-consuming process of the Japanese morphological analysis (JMA) was found to be accessing the dictionary stored in a secondary memory. Therefore greater effort was put into designing the dictionary data structure and methods for quick access. The situation, however, has changed. Highly efficient data structures based on the TRIE structure seem to have finally solved the data structure problems (for instance, Morimoto and Aoe, 1993), and the access problem is also being resolved by the emergence of cheap main memory on which the dictionary can be stored directly, and a dictionary-accessing chip that can access the dictionary thousands of times faster (Fukushima, 1991). As a result, problem of treating Japanese verb inflection is becoming more import,ant. Although phonological description of Japanese verb inflection is highly simple, it cannot be applied to JMA directly. Because each Japanese hiragana phonogram basically corresponds to a consonant-vowel pair, not to a phoneme. On the other hand, traditional school grammar gives a description based on the ordinary Japanese writing system, and has thus been widely used in JMA. However it is neither as rational as the phonological description nor is it the most efficient from a computational viewpoint. We reconsider lexical entries for verb inflection and propose a new method for segmenting verbal complexes. Though our method is based on the ordinary Japanese writing system, it has various advantages over existing ones: 1) it minimizes tile number of lexical entries together with avoiding useless segmentation; 2) it requires 20 to 40% less chart parsing computation, where the parser is based on dynamic programming and suitable for robust analysis; 3) it is also suitable for error correction in OCRs; 4) it requires a smaller incident matrix than other treatments, making the morphological analyzer easier to construct and maintain. Section 1 overviews descriptions of Japanese verb inflection in terms of phonology and in terms of traditional school grammar. Section 2 reviews three different treatments of verb inflection in NLP, which are based on the two descriptions in section 1. Section 3 introduces our proposed treatment, and section 4 shows the advantages of our treatment from several aspects, including a quantitative comparison of the computational efficiency of a chart parser.