1. J. Schwinger, Phys. Rev. 73, 407 (1948)
2. N. Akchurin, A. Bravar, M. Conte, A. Penzo, University of Iowa Report 93-04
3. A. Bravar, in Proceedings of the XVI International Symposium on High Energy Spin Physics, 10-16 October 2004, Trieste, edited by K. Aulenbacher, F. Bradamante, A. Bressan, A. Martin, p. 700, ISBN 981-256-315-6 (1995)
4. A. Bravar, to be published in Proceedings of the X EDS International Conference, 23-30 May 2005, Blois, France
5. C. Bourrely, J. Soffer, T.T. Wu, Phys. Rev. D 19, 3249 (1979).