Characterizations of Classes of Programs by Three-Valued Operators

Several important classes of normal logic programs includ ing the classes of acyclic acceptable and locally hierarchical programs have the property that every program in the class has a unique two valued supported model In this paper we call such classes unique sup ported model classes We analyse and characterize these classes by means of operators on three valued logics Our studies will motivate the de ni tion of a larger unique supported model class which we call the class of accessible programs Finally we show that the class of accessible programs is computationally adequate in that every partial recursive function can be implemented by such a program Proceedings of the th International Conference on Logic Programmingand Non Monotonic Reasoning LPNMR El Paso Texas December Springer Lecture Notes in Arti cial Intelligence Vol pp