This work is licensed under the Creative Commons Attribution License. NP-Logic Systems and Model-Equivalence Reductions ∗

In this paper we investigate the existence of model-equivalence reduction between NP-logic systems which are logic systems with model existence problem in NP. It is shown that among all NP-systems with model checking problem in NP, the existentially quantified propositional logic (∃PF) is maximal with respect to poly-time model-equivalent reduction. However, ∃PF seems not a maximal NP-system in general because there exits a NP-system with model checking problem DP-complete.