Resonant Excitation of Disk Oscillations in Deformed Disks IV: A New   Formulation Studying Stability
  The possibility has been suggested that high-frequency quasi-periodic oscillations observed in low-mass X-ray binaries are resonantly excited disk oscillations in deformed (warped or eccentric) relativistic disks (Kato 2004). In this paper we examine this wave excitation process from a viewpoint somewhat different from that of previous studies. We study how amplitudes of a set of normal mode oscillations change secularly with time by their mutual couplings through disk deformation. As a first step, we consider the case where the number of oscillation modes contributing to the resonance coupling is two. The results show that two prograde oscillations interacting through disk deformation can grow if their wave energies have opposite signs. 
Shoji Kato, Atsuo T. Okazaki, and Finny Oktariani
astro-ph.HE