Lexical-Functional Transfer: A Transfer Framework in a Machine Translation System Based on LFG

Lexieal-Functional Transfer: A Transfer Framework in a Machine Translation System Based on LFG Ikuo KUDO CSK Research Institute 3-22-17 Higashi-Ikebukuro, Toshima-ku, Tokyo, 170, Japan Abstract This paper presents a transfer framework called LFT (Lexical-functional Transfer) for a machine translation system based on LFG (Lexical-functional Grammar). The translation process consists of subprocesses of analysis, transfer and generation. We adopt the so called f- structures of LFG as the intermediate representations or interfaces between those subprocesses, thus the transfer process converts a source f-structure into a target f- structure. Since LFG is a grammatical framework for sentence structure analysis of one language, for the purpose, we propose a new framework for specifying transfer rules with LFG schemata, which incorporates corresponding lexical functions of two different languages into an equational representation. The transfer process, therefore, is to solve equations called target f-descriptions derived from the transfer rules applied to the source f- structure and then to produce a target f-structure. 1. Introduction A grammatical theory called LFG (Lexical-functional Grammar)Ill is a framework for sentence structure analysis and has a simple framework for representing lexical and grammatical information. It analyzes a sentence in two steps, a phrase structure analysis and a functional structure analysis. The former is a syntactic analysis and produces constituent structures (c- structures). The latter consists of several procedures, attaching lexical functions to components in the c- structure, deriving flmctional equations called functional descriptions (f-descriptions) from them with preserving configurational relationships, and solving these equations to produce a functional structure (f-structure). Those lexical functions are represented by a representative framework called LFG schema. We adopt such LFG schema to a representative framework for a dictionary and rules which define functional correspondences between components of two languages. With them the transfer process can be designed as a simple procedure such that its task is only to solve functional equations of the target language and then produce an f-structure of the target language. We propose such a framework called LFT (Lexical-functional Transfer). It consists of both a representative framework for a two-way dictionary and transfer rules and a processing mechanism of transferring an f-structure of source language into an f-structure of target language. The representative framework is declarative and then easy to manipulate. The procedure is a mathematical processing and thus enough simple and clear in its nature and executable easily. 112 Hirosato NOMURA NTT Basic Research Laboratories Musashino-shi, Tokyo, 180, Japan 2. Overall construction of translation system Figure I shows the global construction of the translation system. The whole process is divided into three subprocesses, analysis, transfer and generation as usual translation systems. The analysis process is nothing but LFG analysis. LFG ana|ysis~" ~" two- way dictiona r y--~ /Generation F ........... 7 7- --F ............ 7 Source , ~ ~ , Target ( Lexical-functional Transfer ) 1) Dictionary looking-up, 2) Conditioning, 3) lnstantiation, 4) Solving target equations Fig. 1 Translation model based on Lexical-functional Transfer The transfer process, LFT converts an f-structure of a source language into a corresponding f-structure of …

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