Arithmetic classification of perfect models of stratified programs

We study here the recursion theoretic complexity of the perfect (Herbrand) models of stratified logic programs. We show that these models lie arbitrarily high in the arithmetic hierarchy. As a byproduct we obtain a similar characterization of the recursion theoretic complexity of the set of consequences in a number of formalisms for non-monotonic reasoning. We show that under some circumstances this complexity can be brought down to recursiveness and recursive enumerability. To this purpose we study a class of recursion-free programs.

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