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DOI: 10.4230/LIPIcs.SAT.2022.22

QBF Merge Resolution Is Powerful but Unnatural

Authors: Meena Mahajan and Gaurav Sood

Published in: LIPIcs, Volume 236, 25th International Conference on Theory and Applications of Satisfiability Testing (SAT 2022)

Abstract

The Merge Resolution proof system (M-Res) for QBFs, proposed by Beyersdorff et al. in 2019, explicitly builds partial strategies inside refutations. The original motivation for this approach was to overcome the limitations encountered in long-distance Q-Resolution proof system (LD-Q-Res), where the syntactic side-conditions, while prohibiting all unsound resolutions, also end up prohibiting some sound resolutions. However, while the advantage of M-Res over many other resolution-based QBF proof systems was already demonstrated, a comparison with LD-Q-Res itself had remained open. In this paper, we settle this question. We show that M-Res has an exponential advantage over not only LD-Q-Res, but even over LQU^+-Res and IRM, the most powerful among currently known resolution-based QBF proof systems. Combining this with results from Beyersdorff et al. 2020, we conclude that M-Res is incomparable with LQU-Res and LQU^+-Res. Our proof method reveals two additional and curious features about MRes: (i) M-Res is not closed under restrictions, and is hence not a natural proof system, and (ii) weakening axiom clauses with existential variables provably yields an exponential advantage over MRes without weakening. We further show that in the context of regular derivations, weakening axiom clauses with universal variables provably yields an exponential advantage over M-Res without weakening. These results suggest that M-Res is better used with weakening, though whether M-Res with weakening is closed under restrictions remains open. We note that even with weakening, M-Res continues to be simulated by eFrege+∀red (the simulation of ordinary M-Res was shown recently by Chew and Slivovsky).

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Meena Mahajan and Gaurav Sood. QBF Merge Resolution Is Powerful but Unnatural. In 25th International Conference on Theory and Applications of Satisfiability Testing (SAT 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 236, pp. 22:1-22:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{mahajan_et_al:LIPIcs.SAT.2022.22,
  author =	{Mahajan, Meena and Sood, Gaurav},
  title =	{{QBF Merge Resolution Is Powerful but Unnatural}},
  booktitle =	{25th International Conference on Theory and Applications of Satisfiability Testing (SAT 2022)},
  pages =	{22:1--22:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-242-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{236},
  editor =	{Meel, Kuldeep S. and Strichman, Ofer},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SAT.2022.22},
  URN =		{urn:nbn:de:0030-drops-166969},
  doi =		{10.4230/LIPIcs.SAT.2022.22},
  annote =	{Keywords: QBF, proof complexity, resolution, weakening, restrictions}
}

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DOI: 10.4230/LIPIcs.FSTTCS.2020.12

Hard QBFs for Merge Resolution

Authors: Olaf Beyersdorff, Joshua Blinkhorn, Meena Mahajan, Tomáš Peitl, and Gaurav Sood

Published in: LIPIcs, Volume 182, 40th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2020)

Abstract

We prove the first proof size lower bounds for the proof system Merge Resolution (MRes [Olaf Beyersdorff et al., 2020]), a refutational proof system for prenex quantified Boolean formulas (QBF) with a CNF matrix. Unlike most QBF resolution systems in the literature, proofs in MRes consist of resolution steps together with information on countermodels, which are syntactically stored in the proofs as merge maps. As demonstrated in [Olaf Beyersdorff et al., 2020], this makes MRes quite powerful: it has strategy extraction by design and allows short proofs for formulas which are hard for classical QBF resolution systems. Here we show the first exponential lower bounds for MRes, thereby uncovering limitations of MRes. Technically, the results are either transferred from bounds from circuit complexity (for restricted versions of MRes) or directly obtained by combinatorial arguments (for full MRes). Our results imply that the MRes approach is largely orthogonal to other QBF resolution models such as the QCDCL resolution systems QRes and QURes and the expansion systems ∀Exp+Res and IR.

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Olaf Beyersdorff, Joshua Blinkhorn, Meena Mahajan, Tomáš Peitl, and Gaurav Sood. Hard QBFs for Merge Resolution. In 40th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 182, pp. 12:1-12:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{beyersdorff_et_al:LIPIcs.FSTTCS.2020.12,
  author =	{Beyersdorff, Olaf and Blinkhorn, Joshua and Mahajan, Meena and Peitl, Tom\'{a}\v{s} and Sood, Gaurav},
  title =	{{Hard QBFs for Merge Resolution}},
  booktitle =	{40th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2020)},
  pages =	{12:1--12:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-174-0},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{182},
  editor =	{Saxena, Nitin and Simon, Sunil},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2020.12},
  URN =		{urn:nbn:de:0030-drops-132530},
  doi =		{10.4230/LIPIcs.FSTTCS.2020.12},
  annote =	{Keywords: QBF, resolution, proof complexity, lower bounds}
}

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QBF Merge Resolution Is Powerful but Unnatural

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Hard QBFs for Merge Resolution

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