Two Simple Characterizations of Well-Founded Semantics

this paper we show that well-founded models can also be defined as fixed points of a natural program transformation (factorization) which is completely analogous to the transformation used in the definition of stable models and is expressed entirely in terms of classical, 2-valued logic. Subsequently, we use this result to provide a constructive definition of well-founded models as fixed points of an iterative factorization procedure. We note that no such constructive characterization is available for stable models which are computationally intractable even in the class of propositional programs [KS89, MT88]. The results obtained in this paper, coupled with our earlier result showing that the wellfounded semantics can be equivalently defined by means of first order completions of logic programs [Prz91c], provide natural and simple characterizations of well-founded semantics, given entirely in terms of classical, 2-valued logic and thus, hopefully, dispel some of th