A fine-grained hierarchy of hard problems in the separated fragment

• Published in: 2017 32nd Annual ACM/IEEE Symposium on Logic in Computer Science (LICS)
• Main Author: Voigt, Marco
• Other Authors: Modeling and Verification of Distributed Algorithms and Systems (VERIDIS), Max-Planck-Institut für Informatik (MPII), Max-Planck-Gesellschaft-Max-Planck-Gesellschaft-Inria Nancy - Grand Est, Institut National de Recherche en Informatique et en Automatique (Inria)-Institut National de Recherche en Informatique et en Automatique (Inria)-Department of Formal Methods (LORIA - FM), Laboratoire Lorrain de Recherche en Informatique et ses Applications (LORIA), Institut National de Recherche en Informatique et en Automatique (Inria)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche en Informatique et en Automatique (Inria)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS)-Laboratoire Lorrain de Recherche en Informatique et ses Applications (LORIA), Institut National de Recherche en Informatique et en Automatique (Inria)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), Max-Planck-Gesellschaft, Joël Ouaknine
• Format: Conference Object
• Language: English
• Published: HAL CCSD 2017
• Subjects: [INFO.INFO-LO]Computer Science [cs]/Logic in Computer Science [cs.LO]; Iceland
• Online Access: https://inria.hal.science/hal-01592172; https://inria.hal.science/hal-01592172/document; https://inria.hal.science/hal-01592172/file/VoigtLICS.pdf; https://doi.org/10.1109/LICS.2017.8005094

International audience Recently, the separated fragment (SF) has been introduced and proved to be decidable. Its defining principle is that universally and existentially quantified variables may not occur together in atoms. The known upper bound on the time required to decide SF's satisfiability problem is formulated in terms of quantifier alternations: Given an SF sentence ∃ z ∀ x1∃ y1. . ∀ xn∃ yn. ψ in which ψ is quantifier free, satisfiability can be decided in non-deterministic n-fold exponential time. In the present paper, we conduct a more fine-grained analysis of the complexity of SF-satisfiability. We derive an upper and a lower bound in terms of the degree ∂ of interaction of existential variables (short: degree) — a novel measure of how many separate existential quantifier blocks in a sentence are connected via joint occurrences of variables in atoms. Our main result is the k-NEXPTIME-completeness of the satisfiability problem for the set of all SF sentences that have degree k or smaller. Consequently, we show that SF-satisfiability is non-elementary in general, since SF is defined without restrictions on the degree. Beyond trivial lower bounds, nothing has been known about the hardness of SF-satisfiability so far.