tailieunhanh - Báo cáo khoa học: "FACTORING RECURSION AND DEPENDENCIES: AN ASPECT OF TREE ADJOINING GRAMMARS (TAG) AND A COMPARISON OF SOME FORMAL PROPERTIES OF TAGS, GPSGS, PLGS, AND LPGS "

There is increasing recognition of the fact that the entire range of dependencies that transformational grammars in their various incarnations h a v e t r i e d t o a c c o u n t f o r c a n be satisfactorily captured by classes of rules that are non-transformational and at the same Clme highly constrlaned in terms of the classes of grammars and languages that they define . | FACTORING RECURSION AND DEPENDENCIES AN ASPECT OF TREE ADJOINING GRAMMARS TAG AND A COMPARISON OF SOME FORMAL PROPERTIES OF TAGS GPSGS PLGS AND LPGS Aravỉnd K. Joshi Department of Computer and Information Science R. 268 Moore School University of Pennsylvania Philadelphia PA 19104 During the last few years there is vigorous activity in constructing highly constrained grammatical systems by eliminating the transformational component either totally or partially. There Is Increasing recognition of the fact that the entire range of dependencies that transformational grammars In their various Incarnations have tried to account for can be satisfactorily captured by classes of rules that are non-transformaclonal and at the same time highly constrlaned in terms of the classes of grammars and languages that they define. Two types of dependencies are especially Important subcategorization and filler-gap dependencies. Moreover these dependencies can be unbounded. One of the motivations for transformations was to account for unbounded dependencies. The so-called non-transformational grammars account for the unbounded dependencies In different ways. In a tree-adjoining grammar TAG which has been introduced earlier In Joshi 1982 unboundedness is achieved by factoring the dependencies and recursion in a novel and we believe in a linguistically Interesting manner. All dependencies are defined on a finite set of basic structures trees which are bounded. Unboundedness is then a corollary of a particular composition operation called ad joining. There are thus no unbounded dependencies in a sense. In this paper we will first briefly describe TAG S which have the following Important properties 1 we can represent the usual transformational relations more or less directly in tag s 2 the power of tag s is only slightly more than that of context-free grammars CFG s In what appears to be just the right way and 3 tag s are powerful enough to characterize dependencies . .