Threshold reactions in crossed constant electromagnetic field with nonzero invariant $E^2-H^2$

Influence of an external static electromagnetic field of special configuration (a superposition of a crossed ($E=H$) and a magnetic field) on certain processes of quantum electrodynamics, nonlinear phenomena in atomic physics and on a number of processes in semiconductor physics has been considered. A fundamental difference in influence produced by the crossed and the magnetic field on the probability of the reactions in question has been. demonstrated. A simple algorithm has been described that allows us to pass from the results valid for nonrelativistic limit in a crossed constant field to the case of the field with nonzero invariant $E^2-H^2\ne0$.