1. C. Alvarez — A.C. Lazer, An application of iopological degree to the periodic competing species problem, J. Austral. Math. Soc. (Ser.B), 28 (1986), 202–218.
2. H. Amann, A note on degree theory for gradient maps, Proc. Amer. Math. Soc, 85 (1982), 591–595.
3. A. Capietto, “Positive periodic solutions”, Magister Ph.Thesis, I.S.A.S., Trieste, 1988, in preparation.
4. L. Cesari, Functional Analysis, nonlinear differential equations and the alternative method, in “Nonlinear Functional Analysis and Differential Equations”, (L. Cesari, R. Kannan and J.D. Schuur, eds), pp.1–197, Dekker, New York, 1977.
5. C. C. Conley, “Isolated invariant sets and the Morse index”, CBMS 38, Amer. Math. Soc, Providence R.I., 1978.