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