Remarks on the method of comparison equations (generalized WKB method)   and the generalized Ermakov-Pinney equation
  The connection between the method of comparison equations (generalized WKB method) and the Ermakov-Pinney equation is established. A perturbative scheme of solution of the generalized Ermakov-Pinney equation is developed and is applied to the construction of perturbative series for second-order differential equations with and without turning points. 
A Kamenshchik, M Luzzi and G Venturi