Quasinormal modes and hidden conformal symmetry in the   Reissner-Nordstrom black hole
  It is shown that the scalar wave equation in the near-horizon limit respects a hidden SL(2,R) invariance in the Reissner-Nordstrom (RN) black hole spacetimes. We use the SL(2,R) symmetry to determine algebraically the purely imaginary quasinormal frequencies of the RN black hole. We confirm that these are exactly quasinormal modes of scalar perturbation around the near-extremal black hole. 
Yong-Wan Kim, Yun Soo Myung, Young-Jai Park
hep-th gr-qc