1. Eiichi Bannai, Osamu Shimabukuro, and Hajime Tanaka, Finite analogues of non-Euclidean spaces and Ramanujan graphs, European J. Combin. 25 (2004), no. 2, 243–259.[MR]
  2. Eiichi Bannai, Osamu Shimabukuro, and Hajime Tanaka, Finite Euclidean graphs and Ramanujan graphs, Discrete Math. 309 (2009), no. 20, 6126–6134.[MR/doi]