A Combinatorial Proof of Ihara-Bass's Formula for the Zeta Function of Regular Graphs

Author Bharatram Rangarajan

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Bharatram Rangarajan

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Bharatram Rangarajan. A Combinatorial Proof of Ihara-Bass's Formula for the Zeta Function of Regular Graphs. In 37th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 93, pp. 46:1-46:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)


We give an elementary combinatorial proof of Bass's determinant formula for the zeta function of a finite regular graph. This is done by expressing the number of non-backtracking cycles of a given length in terms of Chebyshev polynomials in the eigenvalues of the adjacency operator of the graph. A related observation of independent interest is that the Ramanujan property of a regular graph is equivalent to tight bounds on the number of non-backtracking cycles of every length.
  • non-backtracking
  • Ihara zeta
  • Chebyshev polynomial
  • Ramanujan graph
  • Hashimoto matrix


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