By additionally assuming that the perturbation to the gravitational potential is negligible (the Cowling approximation) and that the star's structure varies more slowly with radius than the oscillation mode, the equations can be reduced approximately to one second-order equation for the radial component of the displacement eigenfunction ξ ξ r , d 2 ξ ξ r d r 2 = ω ω 2 c s 2 ( 1 − − N 2 ω ω 2 ) ( S ℓ ℓ 2 ω ω 2 − − 1 ) ξ ξ r where r is the radial co-ordinate in the star, ω ω is the angular frequency of the oscillation mode, c s is the sound speed inside the star, N is the Brunt–Väisälä or buoyancy frequency and S ℓ ℓ is the Lamb frequency. The last two are defined by N 2 = g ( 1 Γ Γ 1 P d p d r − − 1 ρ ρ d ρ ρ d r ) and S ℓ ℓ 2 = ℓ ℓ ( ℓ ℓ + 1 ) c s 2 r 2 respectively. By analogy with the behaviour of simple harmonic oscillators, this implies that oscillating solutions exist when the frequency is either greater or less than both S ℓ ℓ and N . We identify the former case as high-frequency pressure modes (p-modes) and the latter as low-frequency gravity modes (g-modes).