1. A. N. Tikhonov and A. A. Samarskiy, Equation of Mathematical Physics (Nauka, Moscow, 1977) [in Russian].
2. Eigenvalue problems;I. Babuška,1991
3. V. P. Mikhailov, Partial Differential Equations (Nauka, Moscow, 1983) [in Russian].
4. S. P. Strelkov, Introduction into Vibration Theory (Lan’, St. Peterburg, 2005) [in Russian].
5. V. V. Migulin, V. I. Medvedev, E. R. Mustel’, and V. N. Parygin, Foundations of Vibration Theory (Nauka, Moscow, 1988) [in Russian].