Exponential separations using guarded extension variables

TL;DR

This paper investigates the relative strengths of various restricted proof systems in propositional logic, establishing exponential separations and demonstrating polynomial proofs for certain principles, thereby clarifying their comparative power.

Contribution

It provides the first exponential separations among several proof systems without new variables and constructs polynomial-size proofs for the pigeonhole principle in SBC${}^-$.

Findings

Separated RAT${}^-$ from GER${}^-$ and vice versa

Separated SBC${}^-$ from RAT${}^-$

Polynomial-size SBC${}^-$ proofs of the pigeonhole principle

Abstract

We study the complexity of proof systems augmenting resolution with inference rules that allow, given a formula $\Gamma$ in conjunctive normal form, deriving clauses that are not necessarily logically implied by $\Gamma$ but whose addition to $\Gamma$ preserves satisfiability. When the derived clauses are allowed to introduce variables not occurring in $\Gamma$, the systems we consider become equivalent to extended resolution. We are concerned with the versions of these systems without new variables. They are called BC${}^-$, RAT${}^-$, SBC${}^-$, and GER${}^-$, denoting respectively blocked clauses, resolution asymmetric tautologies, set-blocked clauses, and generalized extended resolution. Each of these systems formalizes some restricted version of the ability to make assumptions that hold "without loss of generality," which is commonly used informally to simplify or shorten proofs. Except for SBC${}^-$, these systems are known to be exponentially weaker than extended resolution. They are, however, all equivalent to it under a relaxed notion of simulation that allows the translation of the formula along with the proof when moving between proof systems. By taking advantage of this fact, we construct formulas that separate RAT${}^-$ from GER${}^-$ and vice versa. With the same strategy, we also separate SBC${}^-$ from RAT${}^-$. Additionally, we give polynomial-size SBC${}^-$ proofs of the pigeonhole principle, which separates SBC${}^-$ from GER${}^-$ by a previously known lower bound. These results also separate the three systems from BC${}^-$ since they all simulate it. We thus give an almost complete picture of their relative strengths.