1. E. Schrödinger, Ann. Physik 79, 361 (1926); 80, 437 (1926); 81, 109 (1926)
2. W. Pauli, Z. Physik 43, 601 (1927)
3. J.J. Sakurai, Advanced Quantum Mechanics (Addison-Wesley, Reading, MA, 1967)
4. In the footnote of [3] on page 79, it is noted that within nonrelativistic quantum mechanics, the correct gyromagnetic ratio of $$g = 2$$ is obtained from the kinetic energy operator of (4.76). This was emphasized particularly by Feynman. It is for this reason that this operator is referred to as the Feynman kinetic energy operator $$\hat{T}_{F}$$
5. P.A.M. Dirac, Proc. Roy. Soc. (London) A117, 610 (1927)