The complexity of disjunction in intuitionistic logic

Abstract

We study procedures for the derivability problem of fragments of intuitionistic logic. Intuitionistic logic is known to be PSPACE-complete, with implication being one of the main contributors to this complexity. In fact, with just implication alone, we still have a PSPACE-complete logic. We study fragments of intuitionistic logic with restricted implication and develop algorithms for these fragments which are based on the proof rules. We identify a core fragment whose derivability is solvable in linear time. Adding disjunction elimination to this core gives a logic which is solvable in co-NP. These sub-procedures are applicable to a wide variety of logics with rules of a similar flavour. We also show that we cannot do better than co-NP whenever disjunction elimination interacts with other rules.