1. Atkinson, F.V., andMingarelli, A.B.: Asymptotics of the number of zeros and of the eigenvalues of general weighted Sturm-Liouville problems, J. Reine Angew. Math., 375/6 (1987), 380?393.
2. Azizov, T. Ya., andIokhvidov, I. S.: Linear operators in spaces with an indefinite metric. John Wiley and Sons, Chichester, New York 1989.
3. Beals, R.: Indefinite Sturm-Liouville problems and half-range completeness. J. Differential Equations56 (1985), 391?407.
4. Bognár, J.: Indefinite inner product spaces. Springer-Verlag, Berlin, Heidelberg, New York 1974.
5. Lecture Notes in Mathematics;E.A. Coddington,1981