A dichotomy in the complexity of propositional circumscription

The inference problem for propositional circumscription is known to be highly intractable and, in fact, harder than the inference problem for classical propositional logic. More precisely, in its full generality this problem in /spl Pi//sub 2//sup P/-complete, which means that it has the same inherent computational complexity as the satisfiability problem for quantified Boolean formulas with two alternations (universal-existential) of quantifiers. We use T.J. Schaefer's (1978) framework of generalized satisfiability problems to study the family of all restricted cases of the inference problem for propositional circumscription. Our main result fields a complete classification of the "truly hard"(/spl Pi//sub 2//sup P/-complete) and the "easier" cases of this problem (reducible to the inference problem for classical propositional logic). Specifically, we establish a dichotomy theorem which asserts that each such restricted case is either /spl Pi//sub 2//sup P/-complete or is in co-NP. Moreover, we provide efficiently checkable criteria that tell apart the "truly hard" cases from the "easier" ones.