1. See e.g., L.Z. Fang, R. Ruffini, Basic Concepts in Relativistic Astrophysics (World Scientific, Singapore, 1983)
2. E. Recami, P. Castorina, Lett. Nuovo Cim. 15, 347 (1976)
3. See e.g., R. Alkofer, C.S. Fisher, F.J. Llanes-Estrada, Phys. Lett. B 611, 279 (2005)
4. A. Salam, J. Strathdee, Phys. Rev. D18, 4596 (1978)
5. The Clay Mathematics Institute, Cambridge, MA, has posted a list of Millenium Problems, whose solutions will in each case be awarded with a million dollar prize; problem no. 7 is the proof on color confinement in Yang-Mills Theory