= 0 ~ C2f(r) = 0 (8) where constants Ci and Cz have to be chosen in such a manner that the foregoing two equations have the same solution. Equations (8) are also homogeneous linear differential equations with variable coefficients of the Euler type of second and fourth order, respectively, with a common solution of the form •f(r) = r. Substituting the assumed solution into Eqs. (8), and requiring a common solution, Ci and Cz are found to be i = m + 1 C2 = (m I) (9)