Twin peaks kHz QPOs: mathematics of the 3:2 orbital resonance
  Using the method of multiple scales, one can derive an analytic solution that describes the behaviour of weakly coupled, non-linear oscillations in nearly Keplerian discs around neutron stars or black holes close to the 3:2 orbital epicyclic resonance. The solution obtained agrees with the previous numerical simulation. Such result may be relevant to the kilohertz quasi-periodic variability in X-ray fluxes observed in many Galactic black hole and neutron star sources. With a particular choice of tunable parameters, the solution fits accurately the observational data for Sco X-1. 
Paola Rebusco