In the partial case of a constant electromagnetic field, generated by magnetic charges, when for A$_{µ}$ expression (29): – may be used, the chronological coupling of operators A$_{µ}$ and B$_{µ}$ is equal Similarly, for a constant electromagnetic field, generated by electrical charges, A k µ = − 1 2 F kl $_{A$_{µ}$}$x$_{l}$ = − 1 4 e $^{klmn}$F$_{B}$$_{µ}$$_{mn}$ x$_{l}$ = − 1 4 e klmn ( ∂B$_{µ n}$ ∂x m − ∂B$_{µ m}$ ∂x n ) x$_{l}$ (44) 〈 T { A k $_{µ}$( x ) B q $_{µ}$( y ) } 〉 = − 1 4 e $^{klmn}$x$_{l}$ δ q n ∂D c $_{0}$( x − y ) ∂x m + 1 4 e $^{klmn}$x$_{l}$ δ q m ∂D c $_{0}$( x − y ) ∂x n = 1 2 e $^{klqm}$x$_{l}$ ∂D c $_{0}$( x − y ) ∂x m . (45) 〈 T { A k e ( x ) B q e ( y ) } 〉 = − 1 2 e $^{klqm}$y$_{l}$ ∂D c $_{0}$( x − y ) ∂y m . (46) [1] P. A. M. Dirac, Proc. Roy. Soc., A133, 60 (1931). [2] P. A. M. Dirac, Scientific American, 208, No. 5, 45 (1963). [3] K. A. Milton, Reports on Progress in Physics, 69 (6), 16371711 (2006). [4] F. Rohrlich, Phys. Rev. 150, 1104, (1966) [5] D. Zwanziger, Phys. Rev. 3, 880 (1971). [6] S. Weinberg, Phys. Rev. 138, B988 (1965). [7] L. D. Landau, E. M. Lifshitz, The Classical Theory of Fields , Fourth Edition: Volume 2 (Course of Theoretical Physics Series), (Pergamon Press, Oxford, 2000). [8] N. N. Bogoliubov, D. V. Shirkov, Introduction to the theory of quantized fields , (Wiley, New York, 1980). [9] J. . Schouten, Tensor analysis for physicists , (Clarendon Press, Oxford, 1951). 11