Orthogonality of functions

In general, functions 0(fcfc,/J) and (j) ki,/3) are not orthogonal, inasmuch as they depend on the wave number k of a corresponding area of the medium. For this reason, in order to determine unknown coefficients A and Bm, it is necessary to present one of these as a function of the other. For example, having substituted this expansion of function 4>m ki, P) by functions m kk,P), we obtain an infinite system of equations with an infinite number of unknowns. 

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