Simplified proofs of Weak Normalization for propositional logic

Abstract

We present a new proof of weak normalization for intuitionistic natural deduction. The distinguishing features of this proof are that it works only with cuts rather than cut segments, provides explicit local rules for determining whether to contract a whole proof or reduce one of its subproofs, and in the latter case, which subproof to reduce. We also discuss a formalization of the entire proof in Lean, and present a deterministic algorithm for weak normalization.