[en] Several methods for effective interaction and operator calculations based on the Rayleigh-Schrodinger and Brillouin-Wigner perturbation expansions are studied. Special emphasis is given to the use of Pade approximants for effective operator calculations. The convergence properties of the methods are studied numerically in a Lipkin many-body model. Among the various methods using Pade-approximants, the method based on a variational approach in the BW scheme is found to give most encouraging results for the present model