1. Peletier, L. A., On a class of wave equations, Thesis, Eindhoven, 1967.
2. Broer, L. J. F. andL. A. Peletier, Appl. Sci. Res.17 (1967) 133.
3. Zubov, V. I., Methods of A. M. Lyapunov and their applications, Noordhoff, Groningen 1964.
4. Broer, L. J. F., Proc. 11th Congress of Appl. Mech. Munich 1964. Springer Verlag, Berlin 1966.