Tricolor DAGs for Machine Translation

Tricolor DAGs for Machine Translation Koichi Takeda Tokyo Research Laboratory, IBM Research 1623-14 Shimotsuruma, Yamato, Kanagawa 242, Japan Phone: 81-462-73-4569, 81-462-73-7413 (FAX) t akeda@trl.vnet.ibm.com Abstract Machine translation (MT) has recently been for- mulated in terms of constraint-based knowledge representation and unification theories~ but it is becoming more and more evident that it is not possible to design a practical MT system without an adequate method of handling mismatches be- tween semantic representations in the source and target languages. In this paper, we introduce the idea of "information-based" MT, which is consid- erably more flexible than interlingual MT or the conventional transfer-based MT. Introduction With the intensive exploration of contemporary theories on unification grammars[6, 15, 13] and feature structures[7, 19] in the last decade, the old image of machine translation (MT) as a bru- tal form of natural language processing has given way to that of a process based on a uniform and reversible architecture[16~ 1, 27]. The developers of MT systems based on the constraint-based formalism found a serious prob- lem in "language mismatching," namely, the dif- ference between semantic representations in the source and target languages. 1 Attempts to de- sign a pure interlingual MT system were therefore abandoned, 2 and the notion of "semantic trans- fer"[24, 22] came into focus as a practical so- lution to the problem of handling the language mismatching. The constraint-based formalism[2] seemed promising as a formal definition of trans- fer, but pure constraints are too rigid to be pre- cisely imposed on target-language sentences. Some researchers(e.g., Russell[14]) introduced 1For example, Yasuhara[26] reported there was an overlap of only 10% between his group's English and Japanese concept dictionaries, which covered 0.2 mil- lion concepts. 2Even an MT system with a controlled input language[12] does not claim to be a pure interlingual system. the concept of defeasible reasoning in order to for- malize what is missing from a pure constraint- based approach, and control mechanisms for such reasoning have also been proposed[5. 3]. With this additional mechanism, we can formulate the "transfer" process as a mapping from a set of con- straints into another set of mandatory and defen- sible constraints. This idea leads us further to the concept of "information-based" MT, which means that, with an appropriate representation scheme, a source sentence can be represented by a set of constraints that it implies and that, given a target sentence, the set Co of constraints can be divided into three disjoint subsets: • The subset Co of constraints that is also implied by the target sentence • The subset C+ of constraints that is not im- plied by, but is consistent with, the translated sentence • The subset C- of constraints that is violated by the target sentence The target sentence may also imply another set C~eto of constraints, none of which is in Ca. That is~ the set Ct of constraints implied by the tar- get sentences is a union of C0 and C~e~o, while Cs = CoUC+UC_. When Ca = Co = Ct, …

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