Nonmonotonic Reasoning Is Sometimes Simpler! 1


We establish the complexity of decision problems associated with the nonmonotonic modal logic S4. We prove that the problem of existence of an S4-expansion for a given set A of premises is P 2-complete. Similarly, we show that for a given formula ' and a set A of premises, it is P 2-complete to decide whether ' belongs to at least one S4-expansion for A… (More)