$RETRACTED \, ARTICLE$. It has come to the attention of the Italian Physical Society that this article should not have been published because it repeats work already published in "A new perturbative approach to nonlinear problems" by C. M. Bender, K. A. Milton, S. S. Pinsky, and L. M. Simmons, Jr. in J. Math. Phys. 30 (7), 1447 (1989). Consequently, this paper has been retracted by Il Nuovo Cimento B and should not be cited or referred to in the future.