The epicyclic frequency may equivalently be written formula_43 which shows that it is proportional to the radial derivative of the angular momentum per unit mass, or specific angular momentum. The specific angular momentum must increase outward if stable epicyclic oscillations are to exist, otherwise displacements would grow exponentially, corresponding to instability. This is a very general result known as the "Rayleigh criterion" (Chandrasekhar 1961) for stability. For orbits around a point mass, the specific angular momentum is proportional to formula_44 so the Rayleigh criterion is well satisfied.