N. Andersson et al., GENERALIZED BOHR-SOMMERFELD FORMULA FOR SCHWARZSCHILD BLACK-HOLE NORMAL-MODES, Classical and quantum gravity, 10(4), 1993, pp. 757-765
A generalized Bohr-Sommerfeld-type formula determining the quasinormal
-mode mode frequencies of Schwarzschild black holes is derived using t
he phase-integral method with three transition points. The formula is
valid provided that these points are well separated from each other, a
nd is remarkably accurate for the high-overtone modes. It makes a grea
t improvement over the conventional Bohr-Sommerfeld formula, derived f
rom two transition points.