1. Schwinger, J., Phys. Rev., 1951, vol. 82, p. 664.
2. Biro, T.S., Nielsen, H.B., and Knoll, J., Nucl. Phys. B, 1984, vol. 245, p. 449.
3. Abramovskii, V.A., Gribov, V.N., and Kancheli, O.V., Sov. J. Nucl. Phys., 1974, vol. 18, p. 308.
4. Amelin, N.S., Braun, M.A., and Pajares, C., Phys. Lett. B, 1993, vol. 306, p. 312.
5. Amelin, N.S., Braun, M.A., and Pajares, C., Z. Phys., 1994, vol. 63, p. 507.