69 research outputs found
Evaluating QBF Solvers: Quantifier Alternations Matter
We present an experimental study of the effects of quantifier alternations on
the evaluation of quantified Boolean formula (QBF) solvers. The number of
quantifier alternations in a QBF in prenex conjunctive normal form (PCNF) is
directly related to the theoretical hardness of the respective QBF
satisfiability problem in the polynomial hierarchy. We show empirically that
the performance of solvers based on different solving paradigms substantially
varies depending on the numbers of alternations in PCNFs. In related
theoretical work, quantifier alternations have become the focus of
understanding the strengths and weaknesses of various QBF proof systems
implemented in solvers. Our results motivate the development of methods to
evaluate orthogonal solving paradigms by taking quantifier alternations into
account. This is necessary to showcase the broad range of existing QBF solving
paradigms for practical QBF applications. Moreover, we highlight the potential
of combining different approaches and QBF proof systems in solvers.Comment: preprint of a paper to be published at CP 2018, LNCS, Springer,
including appendi
Lower Bounds for QBFs of Bounded Treewidth
The problem of deciding the validity (QSAT) of quantified Boolean formulas
(QBF) is a vivid research area in both theory and practice. In the field of
parameterized algorithmics, the well-studied graph measure treewidth turned out
to be a successful parameter. A well-known result by Chen in parameterized
complexity is that QSAT when parameterized by the treewidth of the primal graph
of the input formula together with the quantifier depth of the formula is
fixed-parameter tractable. More precisely, the runtime of such an algorithm is
polynomial in the formula size and exponential in the treewidth, where the
exponential function in the treewidth is a tower, whose height is the
quantifier depth. A natural question is whether one can significantly improve
these results and decrease the tower while assuming the Exponential Time
Hypothesis (ETH). In the last years, there has been a growing interest in the
quest of establishing lower bounds under ETH, showing mostly problem-specific
lower bounds up to the third level of the polynomial hierarchy. Still, an
important question is to settle this as general as possible and to cover the
whole polynomial hierarchy. In this work, we show lower bounds based on the ETH
for arbitrary QBFs parameterized by treewidth (and quantifier depth). More
formally, we establish lower bounds for QSAT and treewidth, namely, that under
ETH there cannot be an algorithm that solves QSAT of quantifier depth i in
runtime significantly better than i-fold exponential in the treewidth and
polynomial in the input size. In doing so, we provide a versatile reduction
technique to compress treewidth that encodes the essence of dynamic programming
on arbitrary tree decompositions. Further, we describe a general methodology
for a more fine-grained analysis of problems parameterized by treewidth that
are at higher levels of the polynomial hierarchy
On the complexity of resolution-based proof systems
Propositional Proof Complexity is the area of Computational Complexity that studies the length of proofs in propositional logic. One of its main questions is to determine which particular propositional formulas have short proofs in a given propositional proof system. In this thesis we present several results related to this question, all on proof systems that are extensions of the well-known resolution proof system.
The first result of this thesis is that TQBF, the problem of determining if a fully-quantified propositional CNF-formula is true, is PSPACE-complete even when restricted to instances of bounded tree-width, i.e. a parameter of structures that measures their similarity to a tree. Instances of bounded tree-width of many NP-complete problems are tractable, e.g. SAT, the boolean satisfiability problem. We show that this does not scale up to TQBF. We also consider Q-resolution, a quantifier-aware version of resolution. On the negative side, our first result implies that, unless NP = PSPACE, the class of fully-quantified CNF-formulas of bounded tree-width does not have short proofs in any proof system (and in particular in Q-resolution). On the positive side, we show that instances with bounded respectful tree-width, a more restrictive condition, do have short proofs in Q-resolution. We also give a natural family of formulas with this property that have real-world applications.
The second result concerns interpretability. Informally, we say that a first-order formula can be interpreted in another if the first one can be expressed using the vocabulary of the second, plus some extra features. We show that first-order formulas whose propositional translations have short R(const)-proofs, i.e. a generalized version of resolution with DNF-formulas of constant-size terms, are closed under a weaker form of interpretability (that with no extra features), called definability. Our main result is a similar result on interpretability. Also, we show some examples of interpretations and show a systematic technique to transform some Sigma_1-definitions into quantifier-free interpretations.
The third and final result is about a relativized weak pigeonhole principle. This says that if at least 2n out of n^2 pigeons decide to fly into n holes, then some hole must be doubly occupied. We prove that the CNF encoding of this principle does not have polynomial-size DNF-refutations, i.e. refutations in the generalized version of resolution with unbounded DNF-formulas. For this proof we discuss the existence of unbalanced low-degree bipartite expanders satisfying a certain robustness condition
Parallel Model Counting with CUDA: Algorithm Engineering for Efficient Hardware Utilization
Propositional model counting (MC) and its extensions as well as applications in the area of probabilistic reasoning have received renewed attention in recent years. As a result, also the need for quickly solving counting-based problems with automated solvers is critical for certain areas. In this paper, we present experiments evaluating various techniques in order to improve the performance of parallel model counting on general purpose graphics processing units (GPGPUs). Thereby, we mainly consider engineering efficient algorithms for model counting on GPGPUs that utilize the treewidth of a propositional formula by means of dynamic programming. The combination of our techniques results in the solver GPUSAT3, which is based on the programming framework Cuda that -compared to other frameworks- shows superior extensibility and driver support. When combining all findings of this work, we show that GPUSAT3 not only solves more instances of the recent Model Counting Competition 2020 (MCC 2020) than existing GPGPU-based systems, but also solves those significantly faster. A portfolio with one of the best solvers of MCC 2020 and GPUSAT3 solves 19% more instances than the former alone in less than half of the runtime
Efficient local search for Pseudo Boolean Optimization
Algorithms and the Foundations of Software technolog
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