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Báo cáo khoa học: "THE FORMAL CONSEQUENCES OF USING VARIABLES IN CCG CATEGORIES"
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Combinatory Categorial Grammars, CCGs, (Steedman 1985) have been shown by Weir and loshi (1988) to generate the same class of languages as Tree-Adjoining Grammars (TAG), Head Grammars (HG), and Linear Indexed Grammars (LIG). In this paper, I will discuss the effect of using variables in lexical category assignments in CCGs. It will be shown that using variables in lexical categories can increase the weak generative capacity of CCGs beyond the class of grammars listed above. | THE FORMAL CONSEQUENCES OF USING VARIABLES IN CCG CATEGORIES Beryl Hoffman Dept of Computer and Information Sciences University of Pennsylvania Philadelphia PA 19104 hoffman@linc.cis.upenn.edu Abstract Combinatory Categorial Grammars CCGs Steedman 1985 have been shown by Weir and Joshi 1988 to generate the same class of languages as Tree-Adjoining Grammars TAG Head Grammars HG and Linear Indexed Grammars LIG . In this paper I will discuss the effect of using variables in lexical category assignments in CCGs. It will be shown that using variables in lexical categories can increase the weak generative capacity of CCGs beyond the class of grammars listed above. A Formal Definition for CCGs In categorial grammars grammatical entities are of two types basic categories and functions. A basic category such as NP serves as a shorthand for a set of syntactic and semantic features. A category such as S NP is a function representing an intransitive verb the function looks for an argument of type NP on its left and results in the category s. A small set of combinatory rules serve to combine these categories while preserving a transparent relation between syntax and semantics. Application rules allow functions to combine with their arguments while composition rules allow two functions to combine together. Based on the formal definition of CCGs in Weir-Joshi 1988 a CCG G is denoted by Up Vn S f R where Vt is a finite set of terminals Vn is a finite set of nonterminals s is a distinguished member of Vjv f is a function that maps elements of Vr u e to finite subsets of C V v the set of categories where - Uv G C Viv and - if Cl and C2 G C VN then c c2 and ci c2 G C VN . 1 would like to thank Mark Steedman Libby Levison Owen Rambow and the anonymous referees for their valuable advice. This work was partially supported by DARPA N00014-90-J-1863 ARO D A AL03-89-C-0031 NSFIRI90-16592 Ben Franklin 91S.3078C-1. R is a finite set of combinatory rules where X Y Z . Zn are variables over .