1. [1] G. W. Benthien, M. E. Gurtin, & T. D. Ralston, On the semi-discrete Galerkin method for hyperbolic problems and its application to problems in elasto-dynamics, Arch. Rational Mech. Anal., 48 (1972), 51-63.
2. [2] T. Dupont, L2 estimates for Galerkin methods for second order hyperbolic equa-tions, SIAM J. Numer. Anal., 10 (1973), 880-889.
3. [3] H. Fujii, Finite element schemes: stability and convergence, Advances in com-putational methods in structural mechanics and design (Proceedings of second U.S.-Japan seminar on matrix methods of structural analysis and design), (1972), 201-218.
4. [4] T. Kato, Perturbation theory for linear operators, Chapter IX, Springer-Verlag, Berlin, 1966.
5. [5] S. G. Krein, Linear differential equations in Banach space, Russian original, Nauka, Moscow, 1967, Japanese translation, Yoshioka-Shoten, Kyoto, 1972.