Number of Variables for Graph Differentiation and the Resolution of GI Formulas

Authors Jacobo Torán , Florian Wörz

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  • 18 pages

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Author Details

Jacobo Torán
  • Universität Ulm, Germany
Florian Wörz
  • Universität Ulm, Germany


We thank Pascal Schweitzer for helpful discussions. We would also like to thank the anonymous reviewers for many insightful comments and suggestions.

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Jacobo Torán and Florian Wörz. Number of Variables for Graph Differentiation and the Resolution of GI Formulas. In 30th EACSL Annual Conference on Computer Science Logic (CSL 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 216, pp. 36:1-36:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


We show that the number of variables and the quantifier depth needed to distinguish a pair of graphs by first-order logic sentences exactly match the complexity measures of clause width and positive depth needed to refute the corresponding graph isomorphism formula in propositional narrow resolution. Using this connection, we obtain upper and lower bounds for refuting graph isomorphism formulas in (normal) resolution. In particular, we show that if k is the number of variables needed to distinguish two graphs with n vertices each, then there is an n^O(k) resolution refutation size upper bound for the corresponding isomorphism formula, as well as lower bounds of 2^(k-1) and k for the tree-like resolution size and resolution clause space for this formula. We also show a (normal) resolution size lower bound of exp(Ω(k²/n)) for the case of colored graphs with constant color class sizes. Applying these results, we prove the first exponential lower bound for graph isomorphism formulas in the proof system SRC-1, a system that extends resolution with a global symmetry rule, thereby answering an open question posed by Schweitzer and Seebach.

Subject Classification

ACM Subject Classification
  • Theory of computation → Proof complexity
  • Theory of computation → Complexity theory and logic
  • Mathematics of computing → Graph theory
  • Proof Complexity
  • Resolution
  • Narrow Width
  • Graph Isomorphism
  • k-variable fragment first-order logic 𝔏_k
  • Immerman’s Pebble Game
  • Symmetry Rule
  • SRC-1


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