1. See, for example, the review article,G. Feinberg:Proc. Roy. Soc.,285 A, 257 (1965).
2. D. Boccaletti, V. De Sabbata andC. Gualdi:Nuovo Cimento,33, 520 (1964).
3. M. Gell-Mann:Phys. Rev. Lett.,6, 70 (1961).
4. S. G. Matinyan andN. N. Tsilosuni:Žurn. Ėksp. Teor. Fiz.,41, 1681 (1961);Sov. Phys. JETP,14, 1195 (1961).
5. R. P. Feynman andM. Gell-Mann:Phys. Rev.,109, 193 (1958).