1. V. M. Pivovarchik, “A boundary value problem connected with the oscillation of elastic beams with internal and viscous damping,”Moscow Univ. Math. Bulletin. Ser. 1, No. 3, 68–71 (1987).
2. T. Kato,Perturbation Theory for Linear Operators, 2nd edition, Springer-Verlag, New York (1976).
3. P. Lancaster and A. A. Shkalikov, “Damped vibrations of beams and related spectral problems,” Research Report No. 741. Dept. of Math. and Stat. Univ. of Calgary (1992) (to appear in Canad. Appl. Math. Quart.).
4. P. Lancaster, A. A. Shkalikov, and Qiang Ye, “Strongly definitizable linear pencils in Hilbert space,”Integr. Equat. Oper. Theory,17, 338–360 (1993).
5. M. V. Keldysh, “On the completeness of eigenfunctions of certain classes of non-selfadjoint linear operators,”Usp. Mat. Nauk,26, No. 4, 15–41 (1971).