Variational perturbation method and power-series approximation method

The power-series approximation method for solving regular perturbation problems is reexamined to show why the method works. As another way to approach these problems, the variational perturbation technique is described. Although the assumptions on which each method is based and the mechanism of deriving their differential equations are different, the system of differential equations developed by each method is identical.Dedicated to R. Bellman