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Chris Godsil
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For regular graphs the Ihara zeta function is determined by the spectrum of the adjacency matrix, and so graphs can have the same zeta function and different chromatic number. For examples take the complements of the Shrikande graph and the line graph of $K_{4,4}$ (which have chromatic numbers 6 and 4 respectively.

Chris Godsil
  • 12.1k
  • 5
  • 37
  • 61