The complexity of propositional proofs
  In this paper, we consider the complexity of propositional proofs of classical and intuitionistic tautologies. In fact, we describe a nondeterministic polynomial-time decision procedure for intuitionistic implicational tautologies. For this purpose, we reduce a decision problem for intuitionistic implicational tautologies to a decision problem for deductive proof diagrams which are a short form for representing of proofs in the intuitionistic implicational calculus. Next, we transform deductive proof diagrams to a special form in which any proof has the size bounded by a polynomial in the length of input formula. Also, we show that this procedure can be extended to all classical and intuitionistic tautologies, and deduce some corollaries including results about complexity classes and polynomially bounded proof systems. 
Grigoriy V. Bokov
math.LO