  High-frequency quasi-periodic oscillations detected in the light curves of
black hole candidates can, according to one model, be identified with
hydrodynamic oscillations of the accretion disc. We describe a non-linear
coupling mechanism, suggested by Kato, through which inertial waves trapped in
the inner regions of accretion discs around black holes are excited. Global
warping and/or eccentricity of the disc have a fundamental role in this
coupling: they combine with trapped modes, generating negative energy waves,
that are damped as they approach the inner edge of the disc or their corotation
resonance. As a result of this damping, inertial oscillations are amplified. We
calculate the resulting eigenfunctions and their growth rates.
