1. Andronov, A. A. et al.: Qualitative theory of second-order dynamic systems. New York: Wiley 1973
2. Andronov, A. A. et al.: Theory of bifurcations of dynamical systems on a plane. New York: Wiley 1973
3. Chicone, C., Shafer, D.: Separatrix and limit cycles of quadratic systems and a theorem of Dulac. Trans. Am. Math. Soc. 278, 585?612 (1983)
4. Coppel, A.: A survey of quadratic systems. J. Differ. Equations 2, 293?304 (1966)
5. Forker, E. L.: Hepatic transport of organic solutes. In: Handbook of Physiology: The Gastrointestinal System, Sect. 6, Vol. 111, Chap. 34, pp. 693?716. Bethesda: Am. Physiol. Soc. 1989