Worst-Case to Expander-Case Reductions

Authors Amir Abboud , Nathan Wallheimer

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Author Details

Amir Abboud
  • Weizmann Institute of Science, Rehovot, Israel
Nathan Wallheimer
  • Weizmann Institute of Science, Rehovot, Israel


We thank the anonymous reviewers for their helpful comments that helped us improve the paper.

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Amir Abboud and Nathan Wallheimer. Worst-Case to Expander-Case Reductions. In 14th Innovations in Theoretical Computer Science Conference (ITCS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 251, pp. 1:1-1:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


In recent years, the expander decomposition method was used to develop many graph algorithms, resulting in major improvements to longstanding complexity barriers. This powerful hammer has led the community to (1) believe that most problems are as easy on worst-case graphs as they are on expanders, and (2) suspect that expander decompositions are the key to breaking the remaining longstanding barriers in fine-grained complexity. We set out to investigate the extent to which these two things are true (and for which problems). Towards this end, we put forth the concept of worst-case to expander-case self-reductions. We design a collection of such reductions for fundamental graph problems, verifying belief (1) for them. The list includes k-Clique, 4-Cycle, Maximum Cardinality Matching, Vertex-Cover, and Minimum Dominating Set. Interestingly, for most (but not all) of these problems the proof is via a simple gadget reduction, not via expander decompositions, showing that this hammer is effectively useless against the problem and contradicting (2).

Subject Classification

ACM Subject Classification
  • Theory of computation → Design and analysis of algorithms
  • Fine-Grained Complexity
  • Expander Decomposition
  • Reductions
  • Exact and Parameterized Complexity
  • Expander Graphs
  • Triangle
  • Maximum Matching
  • Clique
  • 4-Cycle
  • Vertex Cover
  • Dominating Set


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