1. Bardeen W A, Chanowitz M S, Drell S D, Weinstein M and Yan T M 1975Phys. Rev. D11 1094
2. Baym G and Chin S A 1976Phys. Lett. B62 241
3. Bose S K and Singh L P 1980Phys. Rev. D22 773
4. Callan C G, Dashen R F and Gross D J 1979Phys. Rev. D19 1826
5. Chaplin G and Nauenberg M 1977Phys. Rev. D16 450