Separation of variables in the Klein — Gordon equation. I

All types of external electromagnetic fields containing arbitrary functions which admit of separation of variables in the Klein-Gordon equation by using three first-order differential symmetry operators, and stationary fields admitting separation of variables by using two first- and one second-order differential operators, are found. The curvilinear coordinates in which the variables are divided are presented and the equations are written down in the separated variables.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 66–72, November, 1973.