Abstract not found

We consider the structural descriptions produced by various grammatical formalisms in terms of the complexity of the paths and the relationship between paths in the sets of structural descriptions that each system can generate. In considering the relationship between formalisms, we show that it is useful to abstract away from the details of the formalism, and examln the nature of their derivation process as reflected by properties of their derivation trees. We find that several of the formalisms considered can be seen as being closely related since they have derivation ee sets with the same structure as those produced by Context-Free Grammars. On the basis of this observation, we describe a class of formalisms which we call Linear Context- Free Rewritin Systems, and show they are recognizable in polynomial time and generate only semilinear languages.

There is currently considerable interest among computational linguists in grammatical formalisms with highly restricted generative power. This paper concerns the relationship between the class of string languages generated by several such formalisms viz. Combinatory Categorial Grammars, Head Grammars, Linear Indexed Grammars and Tree Adjoining Grammars. Each of these formalisms is known to generate a larger class of languages than Context-Free Grammars. The four formalisms under consideration were developed independently and appear superficially to be quite different from one another. The result presented in this paper is that all four of the formalisms under consideration generate exactly the same class of string languages. 1 Introduction There is currently considerable interest among computational linguists in grammatical formalisms with highly restricted generative power. This is based on the argument that a grammar formalism should not merely be viewed as a notation, but as part o...

this paper we present a scheme to extend a recognition algorithm for Context-Free Grammars (CFG) that can be used to derive polynomial-time recognition algorithms for a set of formalisms that generate a superset of languages generated by CFG. We describe the scheme by developing a Cocke-Kasami-Younger (CKY)-like pure bottom-up recognition algorithm for Linear Indexed Grammars and show how it can be adapted to give algorithms for Tree Adjoining Grammars and Combinatory Categorial Grammars. This is the only polynomial-time recognition algorithm for Combinatory Categorial Grammars that we are aware of

No assumption is more fundamental in the theory (and practice) of syntax than

this dissertation. I single out for special thanks first a few of the Ventura Hall crowd, including Mfirvet Eng, Nancy Wiegand, Susan Stucky (the other Mennonite formal linguist), and Kathie Carpenter, Suzanne Kemmer and Michael Barlow, with whom I have happily shared every step of the Stanford graduate pilgrimage. Next, I warmly thank Gina Wein for her competent administrative support and for her friendship. Finally, I gratefully acknowledge the strong shaping influences of the members of the Stanford linguistics faculty, who teach and also model a vibrant and professional approach to linguistic research. Representative of these scholars are the three members of my reading committee, whose work and counsel have had a profound effect on my work; I thank Joan Bresnan, Ivan Sag, and my principal advisor, Thomas Wasow, whose patience, cheerful persistence, unstinting support, solid critique, creative ideas, and common sense made the writing of this thesis possible and enjoyable. Every student should have such an advisor

This paper outlines a simple and general notion of syntactic category on a metatheoretical level, independent of the notations and substantive claims of any particular grammatical framework. We define a class of formal objects called "category structures" where each such object provides a constructive definition for a space of syntactic categories. A unification operation and subsumption and identity relations are defined for arbitrary syntactic categories. In addition, a formal language for the statement of constraints on categories is provided. By combining a category structure with a set of constraints, we show that one can define the category systems of several well-known grammatical frameworks: phrase structure grammar, tagmemics, augmented phrase structure grammar, relational grammar, transformational grammar, generalized phrase structure grammar, systemic grammar, categorial grammar, and indexed grammar. The problem' of checking a category for conformity to constraints is shown to be soivable in linear time. This work provides in effect a unitary class of data structures for the representation of syntactic categories in a range of diverse grammatical frameworks. Using such data structures should make it possible for various pseudo-issues in natural language processing research to be avoided. We conclude by examining the questions posed by set-valued features and sharing of values between distinct feature specifications, both of which fall outside the scope of the formal system developed in this paper

The lexicon now plays a central role in our implementation of a Head-driven Phrase Structure Grammar (HPSG), given the massive relocation into the lexicon of linguistic information that was carried by the phrase structure rules in the old GPSG system. HPSG's grammax contains fewer than twenty (very general) rules; its predecessor required over 350 to achieve roughly the same coverage. This simplification of the grammar is made possible by an enrichment of the structure and content of lexical entries, using both inheritance mechanisms and lexical rules to represent thc linguistic information in a general and efficient form. We will argue that our mechanisms for structure-sharing not only provide the ability to express important linguistic generalizations about the lexicon, but also make possible an efficient, readily modifiable implementation that we find quite adequate for continuing development of a large natural language system.

This paper defines multiset-valued linear index gram- mar and unordered vector grammar with dominance links. The former models certain uses of multisetvalued feature structures in unification-based for- malisms, while the latter is motivated by word order variation and by "quasi-trees", a generalization of trees. The two formalisms are weakly equivalent, and an important subset is at most context-sensitive and polyno- mially parsable.

This paper describes an extension of the semantic grammars used in conventional statistical spoken language interfaces to allow the probabilities of derived analyses to be conditioned on the meanings or denotations of input utterances in the context of an interface's underlying application environment or world model. Since these denotations will be used to guide disambiguation in interactive applications, they must be ef- ciently shared among the many possible analyses that may be assigned to an input utterance. This paper therefore presents a formal restriction on the scope of variables in a semantic grammar which guarantees that the denotations of all possible analyses of an input utterance can be calculated in polynomial time, without undue constraints on the expressivity of the derived semantics. Empirical tests show that this model-theoretic interpretation yields a statistically signi cant improvement on standard measures of parsing accuracy over a baseline grammar not conditioned on denotations.