1. A. M. Lyapunov, Stability of Motion, Academic Press, London (1966).
2. J. O. C. Ezeilo, “An instability theorem for a certain fourth order differential equation,” Bull. London Math. Soc., 10, No. 2, 184–185 (1978). —.
3. R. Reissig, G. Sansone, and R. Conti, Non-Linear Differential Equations of Higher Order, Noordhoff, Leyden (1974).
4. J. O. C. Ezeilo, “Extension of certain instability theorems for some fourth and fifth order differential equations,” Atti Accad. Naz. Lincei. Rend. Cl. Sci. Fis., Mat. Natur., 8, No. 4, 239–242 (1979).
5. J. O. C. Ezeilo, “Further instability theorems for some fourth order differential equations,” J. Niger. Math. Soc., 19, 1–7 (2000).