1. D K Choudhury, Pratibha Das, D D Goswami and J K Sarma,Pramana — J. Phys. 44, 519 (1995)
2. A K Ghatak and S Lokanathan, inQuantum mechanics: theory and applications (Macmillan, Madras, 1986) p. 255
3. J J Sakurai, inAdvanced quantum mechanics (Massachusetts, Addison-Wesley Publishing Company, 1967) p. 128
4. C Itzykson and J Zuber, inQuantum field theory (McGraw Hill, Singapore, 1986) p. 79
5. N Isgur and M B Wise,Phys. Lett. B232, 113 (1989);B237, 527 (1990)