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.
@InProceedings{rangarajan:LIPIcs.FSTTCS.2017.46, author = {Rangarajan, Bharatram}, title = {{A Combinatorial Proof of Ihara-Bass's Formula for the Zeta Function of Regular Graphs}}, booktitle = {37th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2017)}, pages = {46:1--46:13}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-055-2}, ISSN = {1868-8969}, year = {2018}, volume = {93}, editor = {Lokam, Satya and Ramanujam, R.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2017.46}, URN = {urn:nbn:de:0030-drops-83861}, doi = {10.4230/LIPIcs.FSTTCS.2017.46}, annote = {Keywords: non-backtracking, Ihara zeta, Chebyshev polynomial, Ramanujan graph, Hashimoto matrix} }
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