1. O. Klein, Z. Phys. 42, 407 (1926)
2. M. Kaku, Quantum Field Theory: A Modern Introduction (Oxford University Press, New York, 1993)
3. M.D. Schwartz, Quantum Field Theory and the Standard Model (Cambridge University Press, Cambridge, UK, 2014)
4. P.A.M. Dirac, Proc. R. Soc. London Math. Phys. Sci. A 117, 610 (1928)
5. P.A.M. Dirac, Proc. R. Soc. London A 155, 447 (1936)