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Complexity of Two-Variable Logic on Finite Trees

Published: 11 November 2016 Publication History

Abstract

Verification of properties expressed in the two-variable fragment of first-order logic FO2 has been investigated in a number of contexts. The satisfiability problem for FO2 over arbitrary structures is known to be NEXPTIME-complete, with satisfiable formulas having exponential-sized models. Over words, where FO2 is known to have the same expressiveness as unary temporal logic, satisfiability is again NEXPTIME-complete. Over finite labelled ordered trees, FO2 has the same expressiveness as navigational XPath, a popular query language for XML documents. Prior work on XPath and FO2 gives a 2EXPTIME bound for satisfiability of FO2 over trees. This work contains a comprehensive analysis of the complexity of FO2 on trees, and on the size and depth of models. We show that different techniques are required depending on the vocabulary used, whether the trees are ranked or unranked, and the encoding of labels on trees. We also look at a natural restriction of FO2, its guarded version,  GF2. Our results depend on an analysis of types in models of FO2 formulas, including techniques for controlling the number of distinct subtrees, the depth, and the size of a witness to satisfiability for FO2 sentences over finite trees.

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      Published In

      cover image ACM Transactions on Computational Logic
      ACM Transactions on Computational Logic  Volume 17, Issue 4
      November 2016
      292 pages
      ISSN:1529-3785
      EISSN:1557-945X
      DOI:10.1145/2996393
      • Editor:
      • Orna Kupferman
      Issue’s Table of Contents
      Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from [email protected]

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      Publication History

      Published: 11 November 2016
      Accepted: 01 September 2016
      Revised: 01 July 2016
      Received: 01 October 2014
      Published in TOCL Volume 17, Issue 4

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      Author Tags

      1. Trees
      2. logic

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      • Engineering and Physical Science Research Council projects “Enforcement of Constraints on XML Streams”
      • Polish National Science Centre
      • DBOnto: Bridging Ontologies and Databases

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      • (2023)A Uniform One-Dimensional Fragment with Alternation of QuantifiersElectronic Proceedings in Theoretical Computer Science10.4204/EPTCS.390.1390(1-15)Online publication date: 30-Sep-2023
      • (2023)Lifted inference with tree axiomsArtificial Intelligence10.1016/j.artint.2023.103997324(103997)Online publication date: Nov-2023
      • (2022)One-dimensional fragment over words and treesJournal of Logic and Computation10.1093/logcom/exac00232:5(902-941)Online publication date: 19-Feb-2022
      • (2019)Decidable XPath Fragments in the Real WorldProceedings of the 38th ACM SIGMOD-SIGACT-SIGAI Symposium on Principles of Database Systems10.1145/3294052.3319685(285-302)Online publication date: 25-Jun-2019

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