1. A. A. Andronov, E. A. Leontovich, I. I. Gordon and A. G. Maier, Qualitative Theory of Second-Order Dynamic Systems, Halstead Press, New York, 1973.
2. G. Belostotski and H. I. Freedman, A control theory model for cancer treatment by radiotherapy, Int. J. Pure Appl. Math., 25 (2005), 447–480.
3. E. A. Coddington and N. Levinson, Theory of Ordinary Differential Equations, McGraw-Hill, New York, 1955.
4. H. I. Freedman, Estimates on the existence region for periodic solutions of equations involving a small parameter. I: The noncritical case, SIAM J. Appl. Math., 16 (1968), 1341–1349.
5. H. I. Freedman and P. Waltman, Persistence in models of three interacting predator-prey populations, Math. Biosci., 68 (1984), 213–231.