“…Usually, if we want to solve a partial differential equation using the Fourier method (i.e., the separation of variables) then we consider the problem of expanding an arbitrary function as a series of eigenfunctions. Hence the eigenfunction expanding problem has been studied extensively in the literature (see [2], [3], [4], [5], [6], [7], [13], [15], [16], [21], [22], [29], [30], [31], [34], [35], [36], [38], [39]).…”