A tower of Ramanujan graphs and a reciprocity law of graph zeta functions

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 (sqrt{l}-1)^{2}/2 from below. We also show graph (or Ihara) zeta functions satisfy a certain reciprocity law.