“…For the sake of definiteness, we shall concentrate on the case h 0 = h 1 = ∞ and h 0 = h ∈ R only, the other cases being analogous. More precisely, we shall formulate necessary and sufficient conditions on sequences (λ n ) and (µ n ) in order that they should be eigenvalues of the Sturm-Liouville operators T (q, ∞, ∞) and T (q, h, ∞), respectively, for some choice of q in W s−1 2 (0, 1) and h ∈ R. For an arbitrary intermediate value s ∈ (0, 1), the direct spectral problem was studied in [10,13,26]. For instance, it was proved in [10] that the eigenvalue remaindersλ n andμ n defined in (1.5) are, respectively, even and odd sine Fourier coefficients of some function from W s 2 (0, 1) (cf.…”