“…In addition, we would like to emphasize that the restriction considered in our paper [12] was not unique, i.e. in many of our previous works [12,[20][21][22][23]27,28] devoted to asymptotic and spectral analysis on non-selfadjoint operators, generated by di erent problems on the vibrations of exible structures, we had to impose similar connections between the boundary parameters and the mechanical and structural parameters of the corresponding models (strings, rods, beams, etc). Moreover, in many papers of di erent authors [16,17,[29][30][31][32], the same type of a restriction on the boundary parameter is imposed.…”