Proof. Suppose that one of the solutions of the above equation is y1 = 1/ζ. By the reduction of order method, we suppose the second solution of the above solution is y2 = y1u(ζ) = u ζ . By taking derivative to this expression, we derive y20 = u0 ζ − u ζ2 and y200 = u00 ζ − 2u0 ζ2 + 2u ζ3 . Substituting these two expressions to equation (9), it yields