A tower of Ramanujan graphs and a reciprocity law of graph zeta functions
Abstract
Let l be an odd prime. We will construct a tower of connected regular Ramanujan graph of degree l+1 from of modular curves. This supplies an example of a collection of graphs whose discrete Cheeger constants are bounded by (sqrtl-1)2/2 from below. We also show graph (or Ihara) zeta functions satisfy a certain reciprocity law.
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