Aravind K. Joshi

Tree adjunct grammars

In this paper, a tree generating system called a tree adjunct grammar is described and its formal properties are studied relating them to the tree generating systems of Brainerd (Information and Control14 (1969), 217-231) and Rounds (Mathematical Systems Theory 4 (1970), 257-287) and to the recognizable sets and local sets discussed by Thatcher (Journal of Computer and System Sciences1 (1967), 317-322; 4 (1970), 339-367) and Rounds. Linguistic relevance of these systems has been briefly discussed also.

PROVIDING A UNIFIED ACCOUNT OF DEFINITE NOUN PHRASES IN DISCOURSE

Citation Grosz, Barbara J., Aravind K. Joshi, and Scott Weinstein. 1983. Providing a unified account of definite noun phrases in discourse. In 21st Annual Meeting of the Association for Computational Linguistics: proceedings of the conference : 15-17 June 1983, Massachusetts Institute of Technology, Cambridge, Massachusetts, ed. Association for Computational Linguistics, 44-50. Morristown, N.J.: Association for Computational Linguistics.

CHARACTERIZING STRUCTURAL DESCRIPTIONS PRODUCED BY VARIOUS GRAMMATICAL FORMALISMS

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 examine 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 tree 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 Rewriting Systems, and show they are recognizable in polynomial time and generate only semilinear languages.

Anaphora and Discourse Structure

We argue in this article that many common adverbial phrases generally taken to signal a discourse relation between syntactically connected units within discourse structure instead work anaphorically to contribute relational meaning, with only indirect dependence on discourse structure. This allows a simpler discourse structure to provide scaffolding for compositional semantics and reveals multiple ways in which the relational meaning conveyed by adverbial connectives can interact with that associated with discourse structure. We conclude by sketching out a lexicalized grammar for discourse that facilitates discourse interpretation as a product of compositional rules, anaphor resolution, and inference.

Processing of sentences with intra-sentential code-switching

Speakers of certain bilingual communities systematically produce utterances in which they switch from one language to another, suggesting that the two language systems systematically interact with each other in the production (and recognition) of these sentences. We have investigated this phenomenon in a formal or computational framework which consists of two grammatical systems and a mechanism for switching between the two systems. A variety of constraints apparent in these sentences are then explained in terms of constraints on the switching mechanism, especially, those on closed class items.

Disambiguation of super parts of speech (or supertags): almost parsing

In a lexicalized grammar formalism such as Lexicalized Tree-Adjoining Grammar (LTAG), each lexical item is associated with at least one elementary structure (supertag) that localizes syntactic and semantic dependencies. Thus a parser for a lexicalized grammar must search a large set of supertags to choose the right ones to combine for the parse of the sentence. We present techniques for disambiguating supertags using local information such as lexical preference and local lexical dependencies. The similarity between LTAG and Dependency grammars is exploited in the dependency model of supertag disambiguation. The performance results for various models of supertag disambiguation such as unigram, trigram and dependency-based models are presented.

Some computational properties of tree adjoining grammars

Tree Adjoining Grammar (TAG) is a formalism for natural language grammars. Some of the basic notions of TAGs were introduced in [Joshi, Levy, and Takahashi 1975] and by [Joshi, 1983]. A detailed investigation of the linguistic relevance of TAGs has been carried out in [Kroch and Joshi, 1985]. In this paper, we will describe some new results for TAGs, especially in the following areas: (1) parsing complexity of TAGs, (2) some closure results for TAGs, and (3) the relationship to Head grammars.

Dynamically altering agent behaviors using natural language instructions

Smart avatars are virtual human representations controlled by real people. Given instructions interactively, smart avatars can act as autonomous or reactive agents. During a real-time simulation, a user should be able to dynamically refine his or her avatar’s behavior in reaction to simulated stimuli without having to undertake a lengthy off-line programming session. In this paper, we introduce an architecture, which allows users to input immediate or persistent instructions using natural language and see the agents’ resulting behavioral changes in the graphical output of the simulation.

Feature structures based Tree Adjoining Grammars

We have embedded Tree Adjoining Grammars (TAG) in a feature structure based unification system. The resulting system, Feature Structure based Tree Adjoining Grammars (FTAG), captures the principle of factoring dependencies and recursion, fundamental to TAGs. We show that FTAG has an enhanced descriptive capacity compared to TAG formalism. We consider some restricted versions of this system and some possible linguistic stipulations that can be made. We briefly describe a calculus to represent the structures used by this system, extending on the work of Rounds, and Kasper [Rounds et al. 1986, Kasper et al. 1986] involving the logical formulation of feature structures.

Factoring Predicate Argument and Scope Semantics: Underspecified Semantics with LTAG

In this paper we propose a compositional semantics for lexicalizedtree-adjoining grammar (LTAG). Tree-local multicomponent derivationsallow separation of the semantic contribution of a lexical item into onecomponent contributing to the predicate argument structure and a secondcomponent contributing to scope semantics. Based on this idea asyntax-semantics interface is presented where the compositionalsemantics depends only on the derivation structure. It is shown that thederivation structure (and indirectly the locality of derivations) allowsan appropriate amount of underspecification. This is illustrated byinvestigating underspecified representations for quantifier scopeambiguities and related phenomena such as adjunct scope and islandconstraints.