New many body perturbation method and the Hubbard model

By adapting the functional derivative method developed by Kadanoff and Baym to the Hubbard model, a new perturbation method is formulated. The unperturbed state is defined by the two equations which yield Hubbard's results, while the remainder is given by functional derivatives of the Green's functions which are shown to generate a complete perturbation series. Advantages of this method are discussed.