1. E. Eichten, K. Gottfried, T. Kinoshita, K.D. Lane and T.-M. Yan, Phys. Rev. D17 (1978) 3090
2. B. Simon, Ann. Phys. (NY) 53 (1970) 76.
3. V.M. Vainberg, V.L. Eletskii and V.S. Popov, Sov. Phys.-JETP 54 (1981) 333.
4. S. Graffi, V. Grecchi and M. Maioli, J. Math. Phys. 20 (1979) 685.
5. V. Privman, Phys. Rev. A 22 (1980) 1333.