Abstract
The aim of this paper is to provide another denotational semantics than the least fixpoint semantics for a logic program with negation.
It takes place in a more general work about three-valued (partial) logic and logic programming. We have developped a logical and algebrical theory which seems well-suited to logic programs with negation. In this theory, we have extended the "consequence" operator of Van Emden and Kowalski associated with a program without negation to a "consequence" operator for programs with negation taking inconsistent programs in account. As the models of a program are exactly the post-fixpoints of this operator, the first provided denotational semantics of a program is the least fixpoint semantics.
As an intersection of three-valued Herbrand models of a program is still a model of this program, this least fixpoint is also the set of all the ground literals of a program which are three-valued logical consequences of this program. Even if the least model is thus unique and defined for every consistent program, it is a partial model and may contain only few literals. A second denotational semantics is given by considering the three-valued Herbrand interpretations as partial functions from the set of ground atoms to the set of the truth values {T, F}; the theory of Manna and Shamir can be adapted in two ways:
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The first one is the notion of optimal fixpoint of the operator.
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The second one seems more interesting for our purpose, since the models of a logic program with negation are exactly the post-fixpoints of the consequence operator. It is the notion of optimal post-fixpoint of our "consequence" operator, which is the optimal model of our program. This notion is about to strengthen the three-valued logic. This optimal model is still unique and defined for every consistent program. It is still a partial model but generally strictly contains the least model, and thus really provides another denotational semantics than the least fixpoint semantics for a program with negation. It may even be more interesting since it contains more informations on the program itself.
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Delahaye, J.P., Thibau, V. (1991). The optimal model of a program with negation. In: van Eijck, J. (eds) Logics in AI. JELIA 1990. Lecture Notes in Computer Science, vol 478. Springer, Berlin, Heidelberg. https://doi.org/10.1007/BFb0018441
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DOI: https://doi.org/10.1007/BFb0018441
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