Ah that's so cool. I've been reading about expander graphs lately. Their interesting properties make them pertinent to lots of questions. In particular, you can

Ah that's so cool. I've been reading about expander graphs lately. Their interesting properties make them pertinent to lots of questions. In particular, you can use expander graphs to cook up error correcting codes and prove the PCP theorem. There's a class of expander graphs called Ramanujan graphs which are characterized by an analogue to the Riemann hypothesis involving a zeta function that counts the prime cycles in a graph.