“…F or some purposes, it is convenient to decompose (15), (16) into two pairs of second order differential equations for each component of general solution of (15), (16): and with initial and boundary conditions given, for instance, in the case of electric potential. The equation (20), apart from (iii), is supplemented by Whereas (22) has to be added with In the theory of differential equations any complete solution of (15), (16) consists of a general solution of homogeneous D' Alembert's equation plus some particular solution of the inhomogeneous one. Thus, we can assume that the same procedure can be applied to its equivalent formulation in form (20)- (23).…”