Flexible Japanese Sentence Compression by Relaxing Unit Constraints

Sentence compression is important in a wide range of applications in natural language processing. Previous approaches of Japanese sentence compression can be divided into two groups. Word-based methods extract a subset of words from a sentence to shorten it, while bunsetsubased methods extract a subset of bunsetsu (where a bunsetsu is a text unit that consists of content words and following function words). Basically, bunsetsu-based methods perform better than word-based methods. However, bunsetsu-based methods have the disadvantage that they cannot drop unimportant words from each bunsetsu because they have to follow constraints under which each bunsetsu is treated as a unit. In this paper, we propose a novel compression method to overcome this disadvantage. Our method relaxes the constraints using Lagrangian relaxation and shortens each bunsetsu if it contains unimportant words. Experimental results show that our method effectively compresses a sentence while preserving its important information and grammaticality. TITLE AND ABSTRACT IN JAPANESE ユニット制約の緩和による柔軟な日本語文圧縮 文圧縮は,自然言語処理の様々なアプリケーションにおいて重要である.日本語文に対する 既存の圧縮手法は二種類に分けられる.単語ベースの手法は文から単語集合を選出し,圧縮 文とする.一方,文節ベースの手法は文から文節集合を選出し,圧縮文とする.基本的には 後者の方が良く機能する.しかし,文節ベースの手法は,文節をユニットとして扱うという 制約があるため,個々の文節を圧縮できない.本稿では,この欠点を克服する新しい圧縮手 法を提案する.提案手法はラグランジュ緩和を用いて上の制約を緩和し,各文節を圧縮する. 実験の結果,提案手法によって原文の情報を多く保持する文法的な圧縮文を生成できること が分かった.