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HUNTER-GATHERER:
Applying Constraint Satisfaction, Branch-and-Bound and Solution Synthesis to Computational Semantics

Stephen Beale
Ph.D. Dissertation
Carnegie Mellon University
Language Technologies Institute
School of Computer Science

Abstract:

This workgif integrates three related AI search techniques and applies the result to processing computational semantics, both in the analysis of source text to discover underlying semantics, as well as in the planning of target text using input semantics. We summarize the approach as ``Hunter-Gatherer'' (HG):

Each of these three general AI techniques will be described. We will look at how how they have been used to solve a variety of problems. These general techniques were extended or used in novel ways in this project. We will describe these extensions in detail and give examples of how they were applied to computational semantic processing. A major contribution of this work will also be in showing how and why Natural Language is a prime candidate for applying these methods, and how they can enable near-linear time processing. As part of this discussion, we will demonstrate the important result that by converting Text Planning to a constraint satisfaction problem, Means-End type planning can be replaced by an efficient constraint-based search through a complex tree. We will examine the results in the light of the Mikrokosmos Machine Translation project. This project is a large-scale Spanish-English MT system implemented at New Mexico State University. We will be able to evaluate the control mechanism presented here against a large corpus of sample texts. In particular, we will show that a search space in the billions or more can be reduced to a few thousand or less, with a corresponding decrease in run-time.

Finally, we try to explicitly uncover the benefits of the Hunter-Gatherer system. We apply HG to several different types of graph coloring constraint problems and examine its behavior. We discuss several properties of HG, including the important notions of soundness and completeness. Finally, we show how, in its most basic sense, HG operates by reducing the dimensionality of a problem, and why this produces near linear-time processing for the kinds of topologies found in problems of computational semantics.





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Steve Beale
Wed Mar 26 09:27:50 MST 1997