[1] DAHIYA R. S., SINGH B.: A Liapunov inequality and nonoscillation theorem for a second order nonlinear differential-difference equation. J. Math. Phys. Sci. 7 (1973), 163-170. MR 0350151

[2] ELIASON S. B.: A Liapunov inequality for a certain second order nonlinear differential equation. J. London Math. Soc. 2 (1970), 461-466. MR 0267191

[3] ELIASON S. B.: Liapunov type inequalities for certain second order functional differential equations. SIAM J. Appl. Mat. 27 (1974), 180-199. MR 0350152

[4] ELIASON S. B. : Distance between zeros of certain differential equations having delayed arguments. Ann. Mat. Pura Appl. 106 (1975), 273-291. MR 0412558 | Zbl 0316.34081

[5] HARTMAN P.: Ordinary Differential Equations. Wiley, New York, 1964. MR 0171038 | Zbl 0125.32102

[6] LIAPUNOV A. M.: Probleme general de la stabilitie du mouvement. Ann. of Math. Stud. 17, Princeton Univ. Press, Princeton, NJ, 1949.

[7] PACHPATTE B. G.: On Liapunov-type inequalities for certain higher order differential equations. J. Math. Anal. Appl. 195 (1995), 527-536. MR 1354560

[8] PARHI N., PANIGRAHI S.: On Liapunov-type inequality for third-order differential equations. J. Math. Anal. Appl. 233 (1999), 445-460. MR 1689641

[9] PARHI N., PANIGRAHI S.: On Liapunov-type inequality for delay-differential equations of third order. Czechoslovak Math. J. (To appear). MR 1905446

[10] PATULA W. T.: On the distance between zeros. Proc. Amer. Math. Soc. 52 (1975 , 247-251. MR 0379986