Key points are not available for this paper at this time.
The covariant S-matrix formalism of Dyson has been applied to the calculation of the fourth order non-linear polarization of the vacuum, which is related to the lowest order non-linear interaction between electromagnetic fields. The finiteness and the gauge invariance of the interaction are exhibited explicitly by an expression for the fourth-rank vacuum polarization tensor in momentum space.
Karplus et al. (Wed,) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: