Studies in Semi-Admissible Heuristics

  • Authors:
  • Judea Pearl;Jin H. Kim

  • Affiliations:
  • SENIOR MEMBER, IEEE, Cognitive Systems Laboratory, School of Engineering and Applied Science, University of California, Los Angeles, CA 90024.;Hughes Research Laboratories, Malibu, CA 90265.

  • Venue:
  • IEEE Transactions on Pattern Analysis and Machine Intelligence
  • Year:
  • 1982

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Abstract

The paper introduces three extensions of the A* search algorithm which improve the search efficiency by relaxing the admissibility condition. 1) A* employs an admissible heuristic function but invokes quicker termination conditions while still guaranteeing that the cost of the solution found will not exceed the optimal cost by a factor greater than 1 + . 2) R驴* may employ heuristic functions which occasionally violate the admissibility condition, but guarantees that at termination the risk of missing the opportunity for further cost reduction is at most 驴. 3) R驴*,* is a speedup version of R驴*, combining the termination condition of A* with the risk-admissibility condition of R驴*. The Traveling Salesman problem was used as a test vehicle to examine the performances of the algorithms A* and R驴*. The advantages of A* are shown to be significant in difficult problems, i.e., problems requiring a large number of expansions due to the presence of many subtours of roughly equal costs. The use of R驴* is shown to produce a 4:1 reduction in search time with only a minor increase in final solution cost.