tailieunhanh - Báo cáo khoa học: "A LOGICAL SEMANTICS FOR FEATURE STRUCTURES"

Unification-based grammar formalisms use structures containing sets of features to describe linguistic objects. Although computational algorithms for unification of feature structures have been worked out in experimental research, these algcwithms become quite complicated, and a more precise description of feature structures is desirable. We have developed a model in which descriptions of feature structures can be regarded as logical formulas, and interpreted by sets of directed graphs which satisfy them. These graphs are, in fact, transition graphs for a special type of deterministic finite automaton. . | A LOGICAL SEMANTICS FOR FEATURE STRUCTURES Robert T. Kasper and William c. Rounds Electrical Engineering and Computer Science Department University of Michigan Ann Arbor Michigan 48109 Abstract Unification-based grammar formalisms use structures containing sets of features to describe linguistic objects. Although computational algorithms for unification of feature structures have been worked out in experimental research these algorithms become quite complicated and a more precise description of feature structures is desừ-able. We have developed a model in which descriptions of feature structures can be regarded as logical formulas and interpreted by sets of directed graphs which satisfy them. These graphs are in fact transition graphs for a special type of deterministic finite automaton. This semantics for feature structures extends the ideas of Pereừa and Shieber 11 by providing an interpretation for values which are specified by disjunctions and path values embedded within disjunctions. Our interpretation differs from that of Pereừa and Shieber by using a logical model in place of a denotational semantics. This logical model yields a calculus of equivalences which can be used to simplify formulas. Unification is attractive because of its generality but it is often computationally inefficient. Our model allows a careful examination of the computational complexity of unification. We have shown that the consistency problem for formulas with disjunctive values is NP-complete. To deal with this complexity we describe how disjunctive values can be specified in a way which delays expansion to disjunctive normal form. 1 Background Unification in Grammar Several different approaches to natural language grammar have developed the notion of feature structures to describe linguistic objects. These approaches include linguistic theories such as Generalised Phrase Structure Grammar GPSG 2 Lexical Functional Grammar LFG 4 and Sys temic Grammar 3 . They also include grammar .