Normal proofs and tableaux for the Font-Rius tetravalent modal logic

Abstract

Tetravalent modal logic (T ML) was introduced by Font and Rius in 2000; and it is an expansion of the Belnap-Dunn four{valued logic FOUR, a logical system that is well{known for the many applications it has been found in several fields. Besides, T ML is the logic that preserve degrees of truth with respect to Monteiro's tetravalent modal algebras. Among other things, Font and Rius showed that T ML has a strongly adequate sequent system, but unfortunately this system does not enjoy the cut-elimination property. However, in [16], it was presented a sequent system for T ML with the cut{elimination property. Besides, in this same work, it was also presented a sound and complete natural deduction system for this logic. In the present work, we continue with the study of T ML under a proof-theoretic perspective. In first place, we show that the natural deduction system given in [16] admits a normalization theorem. In second place, taking advantage of the contrapositive implication for the tetravalent modal algebras introduced in [17], we define a decidable tableau system adequate to check validity in the logic T ML. Finally, we provide a sound and complete tableau system for T ML in the original language.These two tableau systems constitute new (proof-theoretic) decision procedures for checking validity in the variety of tetravalent modal algebras.