Signatures, lifts, and eigenvalues of graphs
  We study the spectra of cyclic signatures of finite graphs and the corresponding cyclic lifts. Starting from a bipartite Ramanujan graph, we prove the existence of an infinite tower of $3$-cyclic lifts, each of which is again Ramanujan. 
Shiping Liu, Norbert Peyerimhoff and Alina Vdovina
math.CO cs.DM