where T is the temperature, S is the entropy, C is the specific heat, ρ ρ s is the superfluid density and ρ ρ n is the normal fluid density. As T → → 0 , c 2 = c / 3 , where c = ( ∂ ∂ p / ∂ ∂ ρ ρ ) S ≈ ≈ ( ∂ ∂ p / ∂ ∂ ρ ρ ) T is the ordinary (or first) sound speed.