1. F. Browder,On continuity of fixed points under deformations of continuous mappings, Summa Brasil. Math.,4 (1960), pp. 183–191.
2. L. Cesari,Asymptotic Behavior and Stability Problems in Ordinary Differential Equations, 2nd Ed., Springer-Verlag, Berlin, 1963.
3. L. Cesari,Functional analysis and periodic solutions of nonlinear differential equations, Contr. Diff. Equ.,1 (1963), pp. 149–187.
4. J. Cronin,The number of periodic solutions of nonautonomous systems, Duke Math. J.,27 (1960), pp. 183–194.
5. J. Cronin,Fixed points and topological degree in nonlinear analysis, Am. Math. Soc., Providence, R.I., 1964.