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Algebraic Approach to Promise Constraint Satisfaction

Published: 14 July 2021 Publication History

Abstract

The complexity and approximability of the constraint satisfaction problem (CSP) has been actively studied over the past 20 years. A new version of the CSP, the promise CSP (PCSP), has recently been proposed, motivated by open questions about the approximability of variants of satisfiability and graph colouring. The PCSP significantly extends the standard decision CSP. The complexity of CSPs with a fixed constraint language on a finite domain has recently been fully classified, greatly guided by the algebraic approach, which uses polymorphisms—high-dimensional symmetries of solution spaces—to analyse the complexity of problems. The corresponding classification for PCSPs is wide open and includes some long-standing open questions, such as the complexity of approximate graph colouring, as special cases.
The basic algebraic approach to PCSP was initiated by Brakensiek and Guruswami, and in this article, we significantly extend it and lift it from concrete properties of polymorphisms to their abstract properties. We introduce a new class of problems that can be viewed as algebraic versions of the (Gap) Label Cover problem and show that every PCSP with a fixed constraint language is equivalent to a problem of this form. This allows us to identify a “measure of symmetry” that is well suited for comparing and relating the complexity of different PCSPs via the algebraic approach. We demonstrate how our theory can be applied by giving both general and specific hardness/tractability results. Among other things, we improve the state-of-the-art in approximate graph colouring by showing that, for any k≥ 3, it is NP-hard to find a (2k-1)-colouring of a given k-colourable graph.

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    cover image Journal of the ACM
    Journal of the ACM  Volume 68, Issue 4
    August 2021
    297 pages
    ISSN:0004-5411
    EISSN:1557-735X
    DOI:10.1145/3468065
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    Publication History

    Published: 14 July 2021
    Accepted: 01 March 2021
    Revised: 01 November 2020
    Received: 01 July 2019
    Published in JACM Volume 68, Issue 4

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    Author Tags

    1. Constraint satisfaction
    2. promise problem
    3. approximation
    4. graph colouring
    5. polymorphism

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    • Refereed

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    • European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programme (CoCoSym, CSP-Infinity)
    • Austrian Science
    • Czech Science Foundation
    • Charles University Research Centre program
    • UK EPSRC

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