Ramanujan graphs by Lubotzky, Phillips and Sarnak. Indeed, the name "Ramanujan graph" was derived from this connection. Another application is that the Ramanujan–Petersson conjecture for the general linear group implies Selberg's conjecture about eigenvalues of the Laplacian for some discrete groups. References Reprinted in Modular forms Zeta and L-functions Srinivasa Ramanujan Conjectures