(1) The second order differential equation ρ((d ²f/dR ²) (6f/R ²)) + (6ñ/R)((df/dR) + (f/R)) = 0 relating the flattening f of a rotational ellipsoidal solid composed of homogeneous, ellipsoidal shells and of average radius R and density ρ(R). Sometimes referred to as Clairaut's equation. It has been generalised (e.g. by Jeffreys) by replacing f by fn, where fn is the n-th coefficient in an expansion of r/R in zonal harmonics. The factor 6 in the equation is then replaced by n(n+1). (2) The first order differential equation y = x(dy/dx) + f(dy/dx). Its solution is.