1. F. Albrecht, H. Gatzke, A. Haddad and N. Wax, “The dynamics of two interacting populations,” J.Math. Anal. Applic., vol. 46, pp. 658–670, 1974. This is the definitive mathematical treatment of systems of the form of (18) under various pertinent hypotheses on f and g. Contains almost none of the biological or ecological interpretations. There is a brief bibliography.
2. W. Baltensweiler, “The relevance of changes in the composition of larch bud-worm populations for the dynamics of its numbers,” in Dynamics of Populations, P. J. den Boer and G. R. Gradwell, Eds. Wageningen: Centre for Agricultural Publishing and Documentation, 1971, pp. 208–219.
3. N. N. Bautin, “On the number of limit cycles which appear with the variation of coefficients from an equilibrium position of focus or center type,” Mat.Sb., vol. 30 (72), pp. 181–196, (1952). English translation in Amer.Math. Soc. Transl., no. 100 (1954). A research level paper containing very detailed analysis.
4. M. G. Bulmer, “A statistical analysis of the 10-year cycle in Canada,” J.Anim. Ecol., vol. 43, pp. 701–718, 1971. A very interesting statistical analysis of the lynx-hare and several other 10-year cycles in the Canadian forests. Discusses 8-year cycles in the Siberian taiga. A very good bibliography.
5. Lan Sun Chen and Ming-Shu Wang, “The relative position and number of limit cycles of a quadratic differential system,” Acta Math. Sinica, vol. 22, pp. 751–758, 1979. A straightforward but somewhat advanced account of the most recent work on quadratic systems with an example showing that H(2) ≧ 4.