“…It is worth mentioning that, in [2,3,14], the coefficients at y (n) and y (n−1) in the differential expression can be arbitrary functions of some classes. In this paper, we confine ourselves to the coefficients 1 and 0 at y (n) and y (n−1) , respectively, because this case is natural for the inverse problem theory (see [1,4,15]).…”