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Bimorphisms and synchronous grammars

Shieber S. M.

Journal of Language Modelling

2014

Vol. 2, No. 1

51--104

We tend to think of the study of language as proceeding by characterizing the strings and structures of a language, and we think of natural-language processing as using those structures to build systems of utility in manipulating the language. But many language-related problems are more fruitfully viewed as requiring the specification of a relation between two languages, rather than the specification of a single language. In this paper, we provide a synthesis and extension of work that unifies two approaches to such language relations: the automata-theoretic approach based on tree transducers that transform trees to their counterparts in the relation, and the grammatical approach based on synchronous grammars that derive pairs of trees in the relation. In particular, we characterize synchronous tree-substitution grammars and synchronous tree-adjoining grammars in terms of bimorphisms, which have previously been used to characterize tree transducers. In the process, we provide new approaches to formalizing the various concepts: a metanotation for describing varieties of tree automata and transducers in equational terms; a rigorous formalization of tree-adjoining and tree-substitution grammars and their synchronous counterparts, using trees over ranked alphabets; and generalizations of tree-adjoining grammar allowing multiple adjunction.

New Grammar Systems and Their Application Perspectives

Horáček P., Meduna A.

Schedae Informaticae

2013

Vol. 22

47--68

This paper presents new grammar systems that describe transfor- mations of syntactic structures. They represent two approaches: synchronous grammars and transducers. The systems consist of well-known models such as context-free grammars and finite automata. Particular attention is paid to synchronization of regulated grammars. The paper recalls formal definitions of the systems and discusses theoretical results regarding their generative and accepting power. The last part briefly introduces application perspectives in natural language translation, illustrated by examples of Czech-English trans- lation.