Summary: We present the covering set of the squared eigenfunctions for a degenerate
eigenvalue problem. The derivation follows the approach recently outlined by us on this same equation [J. Math. Phys. 50, No. 2, Paper No. 023504 (2009; Zbl 1202.35275
)]. This eigenvalue problem is important since it serves as the spectral problem for the inverse scattering transform (IST) of the vector NLS equation, the Sasa-Satsuma equation, and a degenerate two-level Maxwell-Bloch system. The use of this covering set allows one to treat the linear perturbations of these equations in a common and systematic manner. Comparison with previous results on the perturbed continuous spectrum of the Sasa-Satsuma equation is made.