1. Bethe, H. A. and Salpeter, E. E. (1957).Quantum Mechanics of One-and Two-Electron Atoms. Academic Press.
2. Bogoliubov, N. N. and Shirkov, D. V. (1959).Introduction to the Theory of Quantized Fields. Interscience Publishers.
3. Davies, P. C. W. (1970).Proceedings of the Cambridge Philosophical Society,68, 751.
4. Dirac, P. A. M. (1947).Quantum Mechanics. Oxford University Press.
5. Einstein, A. and Mayer, W. (1932).Sitzungsberichte der Akademie der Wissenschaften in Wien,8, 522.