Bogomolny's semiclassical transfer operator for rotationally invariant   integrable systems
  The transfer operator due to Bogomolny provides a convenient method for obtaining a semiclassical approximation to the energy eigenvalues of a quantum system, no matter what the nature of the analogous classical system. In this paper, the method is applied to integrable systems which are rotationally invariant, in two and three dimensions.   In two dimensions, the transfer operator is expanded in a Fourier series in the angle variable, while in three dimensions it is expanded in spherical harmonics. In both cases, when the Fourier coefficients are evaluated using the stationary phase approximation, we arrive at the Einstein-Brillouin-Keller (EBK) quantization conditions.   The associated Maslov indices are shown to agree with the results calculated by well-known simple rules. The theory is applied to several rotationally invariant systems, including the hydrogen atom and the isotropic harmonic oscillator in two and three dimensions, the circle billiard, a billiard inside a spherical cavity, and a harmonic potential with a singular magnetic flux line. 
D. A. Goodings, N. D. Whelan