Near-optimal Lower Bounds on Quantifier Depth and Weisfeiler-Leman Refinement Steps

Christoph Berkholz, Jakob Nordström

Research output: Contribution to journalArticlepeer-review

Abstract

We prove near-optimal tradeoffs for quantifier depth (also called quantifier rank) versus number of variables in first-order logic by exhibiting pairs of n-element structures that can be distinguished by a k-variable first-order sentence but where every such sentence requires quantifier depth at least nω (k/log k). Our tradeoffs also apply to first-order counting logic and, by the known connection to the k-dimensional Weisfeiler-Leman algorithm, imply near-optimal lower bounds on the number of refinement iterations.A key component in our proof is the hardness condensation technique introduced by Razborov in the context of proof complexity. We apply this method to reduce the domain size of relational structures while maintaining the minimal quantifier depth needed to distinguish them in finite variable logics.

Original languageEnglish
Article number32
JournalJournal of the ACM
Volume70
Issue number5
DOIs
Publication statusPublished - 2023 Oct

Subject classification (UKÄ)

  • Algebra and Logic

Free keywords

  • bounded variable fragment
  • first-order counting logic
  • First-order logic
  • hardness condensation
  • lower bounds
  • quantifier depth
  • refinement iterations
  • tradeoffs
  • Weisfeiler-Leman
  • XORification

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