1. Langouche, F., Roekaerts, D., Tirapegui, E.: Functional integration and semiclassical expansions, mathematics and its applications. Vol. 10. Dordrecht: D. Reidel, Publ. Co. 1982
2. Weiss, U.: Z. Phys. B ? Condensed Matter and Quanta30, 429 (1988)
3. Weiss, U.: Phys. Rev. A25, 2444 (1982)
4. Grabert, H., Graham, R., Green, M.S.: Phys. Rev. A21, 2136 (1980)
5. Graham, R.: In: Fluctuations instabilities and phase transitions. Riste, T. (ed.). New York: Plenum Press 1975; Z. Phys. B ? Condensed Matter and Quanta26, 281 (1977);26, 397 (1977), see Eq. (2.9)