1. G. Bethe, S. Schweber, and F.Hofman, Mesons and Fields, Vol. 1, Row, Peterson and Co., Evanston, Ill. (1955).
2. G. F. Chew, M. L. Goldberger, F.E. Low, and Y. Nambu, Phys. Rev., 106 (6): 1345 (1957).
3. J. Ball, Phys. Rev., 124 (6): 2014 (1961).
4. A. I. Lebedev, Dissertation [in Russian], Physics Institute of the Academy of Sciences of the USSR (1964).
5. R. G. Vasil’ev, B. B. Govorkov, and V. I. Gol’danskii, Zh. Eksp. Teor. Fiz., 37(1):11 (1959); B. B. Govorkov, S. P. Denisov, A. I. Lebedev, and E. V. Minarik, Zh. Eksp. Teor. Fiz., 44(5):1463(1963). The paper includes an extensive bibliography on the problem.