Before revision: The second order correction is, for instance n=1, for electron, where the additional approximation of neglecting the perturbation terms due to the bound states with n even and > 2 has been introduced.
After revision: The secokd order correction is, for instance n=1, for electron, where the additional approximation of neglecting the perturbation terms due to the bound states with n even and > 2 has been introduced.