Constraint Satisfaction Search 1 Constraint Satisfaction Problems
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چکیده
For the past thirty ve years artiicial intelligence researchers have been studying heuristic search techniques. People writing AI programs have had strong intuitions about what it means for a program to search. Intuitively they classiied programs according to the kinds of search performed. Certain programs, such as matrix multiplication routines, did no search whatsoever. Other programs searched for solutions to a xed nite set of constraints. Others searched for paths in graphs, or for strategies in game trees, or for proofs in formal inference systems. The intuitive classiication of search programs seems to roughly correspond to complexity classes. The no-search procedures correspond to the complexity class P. Constraint satisfaction search procedures correspond to the complexity class NP, graph search and game search procedures to the class PSPACE, and search in theorem proving to the class of recursively enumerable functions. This chapter discusses heuris-tic techniques for solving search problems of the simplest type | constraint satisfaction problems. Heuristic techniques for other forms of search are discussed in later chapters. Intuitively, a search problem involves nding an object that satisses a given speciication. A classic example is the eight queens problem. This is the problem of placing eight queens on a chess board so that no two queens attack each other, i.e., so that no single row, column, or diagonal contains more than one queen. This problem can be solved ineeciently by a generate and test" program that rst generates a list of all the possible ways eight queens can be placed a chess board and then tests each possibility t o see if it is a solution to the problem.
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