Nonlinear effects in quantum electrodynamics

The fourth-rank vacuum polarization tensor, which is related to the lowest-order nonlinear interaction between four electromagnetic fields in quantum electrodynamics, is exactly calculated in terms of rational, logarithm and dilogarithm functions when two of the four electromagnetic fields describe photons off the mass shell. This task has been accomplished by a not exccedingly laborious effort with the aid of double dispersion relations which...