Equivalent generalized perturbation theory (EGPT)

Based on the well-established generalized perturbation theory (GPT), an equivalent method (EGPT) is proposed. An advantage of this new formulation is that in many circumstances of interest the difficulties in calculating the functions entering into the perturbation expressions can be simplified, in particular, allowing us to reduce the equations governing them from an inhomogeneous to a homogeneous form. After a general derivation of the method, its specialization to a number of different problems is made. Numerical tests for a variety of cases are finally illustrated, confirming the validity of the EGPT methodology.