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08:45-10:15 Session 106A: FLoC Panel (joint with 9 other meetings)
Location: FH, Hörsaal 1
FLoC Panel: Computational Complexity and Logic: Theory vs. Experiments
SPEAKER: unknown

ABSTRACT. The success of SAT solving, Answer Set Programming, and Model Checking has changed our view of computational complexity and (un)decidability. Program committees are increasingly discussing the value of theoretical and empirical complexity evaluations. What is the intellectual value of a completeness result? Which standards do we apply to reproducibility of experiments? What is the role of competitions? What are the minimal requirements for an experimental proof of concept?

10:15-10:45Coffee Break
10:45-13:00 Session 109C: Invited Talk & Higher-Order Logic
Location: FH, Hörsaal 5
And-Or Tableaux for Fixpoint Logics with Converse: LTL, CTL, PDL and CPDL
SPEAKER: Rajeev Gore

ABSTRACT.   Over the last forty years, computer scientists have invented or borrowed
  numerous logics for reasoning about digital systems. Here, I would like to
  concentrate on three of them: Linear Time Temporal Logic (LTL),
  branching time Computation Tree temporal Logic (CTL), and
  Propositional Dynamic Logic (PDL), with and without converse. More
  specifically, I would like to present results and techniques on how
  to solve the satisfiability problem in these logics, with global
  assumptions, using the tableau method. The issues that arise are the
  typical tensions between computational complexity, practicality and
  scalability. This is joint work with Linh Anh Nguyen, Pietro Abate,
  Linda Postniece, Florian Widmann and Jimmy Thomson.

Unified Classical Logic Completeness: A Coinductive Pearl

ABSTRACT. Codatatypes are absent from many programming languages and proof assistants. We make a case for their importance by revisiting a classic result: the completeness theorem for first-order logic established through a Gentzen system. The core of the proof establishes an abstract property of possibly infinite derivation trees, independently of the concrete syntax or inference rules. This separation of concerns simplifies the presentation. The abstract proof can be instantiated for a wide range of Gentzen and tableau systems as well as various flavors of first- order logic. The corresponding Isabelle/HOL formalization demonstrates the recently introduced support for codatatypes and the Haskell code generator.

A Focused Sequent Calculus for Higher-Order Logic

ABSTRACT. We present a focused intuitionistic sequent calculus for higher- order logic. It has primitive support for equality and mixes λ-term con- version with equality reasoning. Classical reasoning is enabled by ex- tending the system with rules for reductio ad absurdum and the axiom of choice. The resulting system is proved sound with respect to Church’s simple type theory. A theorem prover based on bottom up search in the calculus has been implemented. It has been tested on the TPTP higher-order problem set with good results. The problems for which the theorem prover performs best require higher-order unification more fre- quently than the average higher-order TPTP problem. Being strong at higher-order unification the system may serve as a complement to other theorem provers in the field. The theorem prover produces proofs that can be mechanically verified against the soundness proof.

13:00-14:30Lunch Break
14:30-16:00 Session 113C: FLoC Inter-Conference Topic: SAT/SMT/QBF (joint with CAV)
Location: FH, Hörsaal 5
SAT-based Decision Procedure for Analytic Pure Sequent Calculi
SPEAKER: Yoni Zohar

ABSTRACT. We identify a wide family of analytic sequent calculi for propositional non-classical logics whose derivability problem can be uniformly reduced to SAT. The proposed reduction is based on interpreting these calculi using non-deterministic semantics. Its time complexity is polynomial, and, in fact, linear for a useful subfamily. We further study an extension of such calculi with "Next" operators, and show that this extension preserves analyticity and is subject to a similar reduction to SAT. A particular interesting instance of these results is a HORNSAT-based linear-time decision procedure for Gurevich and Neeman's primal infon logic and several natural extensions of it.

A Unified Proof System for QBF Preprocessing
SPEAKER: Marijn Heule

ABSTRACT. For quantified Boolean formulas (QBFs), preprocessing is essential to solve many real-world formulas. Application of a preprocessor, however, prevented the extraction of proofs to independently validate correctness of the solver's result. Especially for universal expansion proof checking was not possible so far. In this paper, we introduce a unified proof system based on three simple and elegant quantified asymmetric tautology QRAT rules. In combination with an extended version of universal reduction, they are sufficient to efficiently express all preprocessing techniques used in state-of-the-art preprocessors including universal expansion. Moreover, these rules give rise to new preprocessing techniques. We equip our preprocessor bloqqer with QRAT proof logging and provide a proof checker for QRAT proofs.

The Fractal Dimension of SAT Formulas

ABSTRACT. Modern SAT solvers have experienced a remarkable progress on solving industrial instances. Most of the techniques have been developed after an intensive experimental testing process. It is believed that these techniques exploit the underlying structure of industrial instances. However, there is not a precise definition of the notion of structure. Recently, there have been some attempts to analyze the structure of these formulas in terms of complex networks, with the long-term aim of explaining the success of these SAT solving techniques, and possibly improving them. We study the fractal dimension of SAT instances with the aim of complementing the model that describes the structure of industrial instances. We show that many industrial families of formulas are self-similar, with a small fractal dimension. We also show how this dimension is affected by the addition of learnt clauses during the execution of SAT solvers.

16:00-16:30Coffee Break
16:30-17:00 Session 116C: FLoC Inter-Conference Topic: SAT/SMT/QBF (joint with CAV)
Location: FH, Hörsaal 5
A Gentle Non-Disjoint Combination of Satisfiability Procedures
SPEAKER: unknown

ABSTRACT. A satisfiability problem is very often expressed in a combination of theories, and a very natural approach consists in solving the problem by combining the satisfiability procedures available for the component theories. This is the purpose of the combination method introduced by Nelson and Oppen. However, in its initial presentation, the Nelson-Oppen combination method requires the theories to be signature-disjoint and stably infinite (to guarantee the existence of an infinite model). The notion of gentle theory has been introduced in the last few years as one solution to go beyond the restriction of stable infiniteness, but in the case of disjoint theories. In this paper, we adapt the notion of gentle theory to the non-disjoint combination of theories sharing only unary predicates (plus constants and the equality). Like in the disjoint case, combining two theories, one of them being gentle, requires some minor assumptions on the other one. We show that major classes of theories, i.e Löwenheim and Bernays-Schönfinkel-Ramsey, satisfy the appropriate notion of gentleness introduced for this particular non-disjoint combination framework.

19:00-21:30 Session 122: VSL Reception 2
Location: University of Vienna, Arkadenhof
22:00-23:59 Session 123: VSL Student Reception 2
Location: Säulenhalle (Volksgarten)