  The results of experimental and theoretical studies of the parametric decay
instability of capillary waves on the surface of superfluid helium He-II are
reported. It is demonstrated that in a system of turbulent capillary waves
low-frequency waves are generated along with the direct Kolmogorov-Zakharov
cascade of capillary turbulence. The effects of low-frequency damping and the
discreteness of the wave spectrum are discussed.
