NONLINEAR-INTERACTION BETWEEN ELECTROMAGNETIC-FIELDS AT HIGH-TEMPERATURE

The electron-positron ''box'' diagram produces an effective action whi ch is fourth order in the electromagnetic held. We examine the behavio r of this effective action at high temperature (in analytically contin ued imaginary-time thermal perturbation theory). We argue that there i s a finite, nonzero limit as T --> infinity (where T is the temperatur e). We calculate this limit in the nonrelativistic static case, and in the long-wavelength limit. We also briefly discuss the self-energy in two-dimensional QED, which is similar in some respects.