NP vs PSPACE

TL;DR

This paper proves that NP equals PSPACE by demonstrating polynomial-size compressed natural deductions for minimal propositional logic tautologies, using a novel horizontal dag-like compression technique.

Contribution

It provides the first proof that NP equals PSPACE through polynomial-size natural deductions, bridging minimal logic and complexity class equivalences.

Findings

Polynomial-size dag-like deductions for minimal tautologies

Horizontal compression reduces tree-like deductions efficiently

Proof runs within Hudelmaier's cut-free sequent calculus

Abstract

We present a proof of the conjecture $\mathcal{NP}$ = $\mathcal{PSPACE}$ by showing that arbitrary tautologies of Johansson's minimal propositional logic admit "small" polynomial-size dag-like natural deductions in Prawitz's system for minimal propositional logic. These "small" deductions arise from standard "large"\ tree-like inputs by horizontal dag-like compression that is obtained by merging distinct nodes labeled with identical formulas occurring in horizontal sections of deductions involved. The underlying "geometric" idea: if the height, $h\left( \partial \right) $ , and the total number of distinct formulas, $\phi \left( \partial \right) $ , of a given tree-like deduction $\partial$ of a minimal tautology $\rho$ are both polynomial in the length of $\rho$, $\left| \rho \right|$, then the size of the horizontal dag-like compression is at most $h\left( \partial \right) \times \phi \left( \partial \right) $, and hence polynomial in $\left| \rho \right|$. The attached proof is due to the first author, but it was the second author who proposed an initial idea to attack a weaker conjecture $\mathcal{NP}= \mathcal{\mathit{co}NP}$ by reductions in diverse natural deduction formalisms for propositional logic. That idea included interactive use of minimal, intuitionistic and classical formalisms, so its practical implementation was too involved. The attached proof of $ \mathcal{NP}=\mathcal{PSPACE}$ runs inside the natural deduction interpretation of Hudelmaier's cutfree sequent calculus for minimal logic.