Max-3-Lin over Non-Abelian Groups with Universal Factor Graphs

Authors Amey Bhangale, Aleksa Stanković

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Amey Bhangale
  • University of California, Riverside, CA, USA
Aleksa Stanković
  • Department of Mathematics, KTH Royal Institute of Technology, Sweden

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Amey Bhangale and Aleksa Stanković. Max-3-Lin over Non-Abelian Groups with Universal Factor Graphs. In 13th Innovations in Theoretical Computer Science Conference (ITCS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 215, pp. 21:1-21:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


Factor graph of an instance of a constraint satisfaction problem with n variables and m constraints is the bipartite graph between [m] and [n] describing which variable appears in which constraints. Thus, an instance of a CSP is completely defined by its factor graph and the list of predicates. We show inapproximability of Max-3-LIN over non-abelian groups (both in the perfect completeness case and in the imperfect completeness case), with the same inapproximability factor as in the general case, even when the factor graph is fixed. Along the way, we also show that these optimal hardness results hold even when we restrict the linear equations in the Max-3-LIN instances to the form x⋅ y⋅ z = g, where x,y,z are the variables and g is a group element. We use representation theory and Fourier analysis over non-abelian groups to analyze the reductions.

Subject Classification

ACM Subject Classification
  • Theory of computation → Problems, reductions and completeness
  • Universal factor graphs
  • linear equations
  • non-abelian groups
  • hardness of approximation


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