Dependency schemes in CDCL-based QBF solving: a proof-theoretic study

Abstract

In Quantified Boolean Formulas QBFs, dependency schemes help to detect spurious or superfluous dependencies that are implied by the variable ordering in the quantifier prefix but are not essential for constructing countermodels. This detection can provably shorten refutations in specific proof systems, and is expected to speed up runs of QBF solvers. The proof system $$\texttt{QCDCL}$$ QCDCL  recently defined by Beyersdorff and Boehm (LMCS 2023) abstracts the reasoning employed by QBF solvers based on conflict-driven clause-learning (CDCL) techniques. We show how to incorporate the use of dependency schemes into this proof system, either in a preprocessing phase, or in the propagations and clause learning, or both. We then show that when the reflexive resolution path dependency scheme $$\texttt{D}^{\texttt{rrs}}$$ D rrs is used, a mixed picture emerges: the proof systems that add $$\texttt{D}^{\texttt{rrs}}$$ D rrs to $$\texttt{QCDCL}$$ QCDCL  in these three ways are not only incomparable with each other, but are also incomparable with the basic $$\texttt{QCDCL}$$ QCDCL  proof system that does not use $$\texttt{D}^{\texttt{rrs}}$$ D rrs at all, as well as with several other resolution-based QBF proof systems. A notable fact is that all our separations are achieved through QBFs with bounded quantifier alternation.