1. A. I. Nikishov, Phys. Part. Nucl. 5, 1466, (2006).
2. J. Schwinger, Particle, Sources, and Fields (Addison-Wesley, New York, 1970).
3. A. I. Nikishov, “On a Phenomenological Approach to Gravity,” Bull. Lebedev Phys. Inst. 45, 360 (2018).
4. S. Weinberg, Gravitation and Cosmology (Wiley, New York, 1972).
5. L. D. Landau and E. M. Lifshitz, Field Theory (Oxford, Pergamon 1977).