1. G. D. Birkhoff, Collected Mathematical Papers New York 1950.
2. Coddington and Levinson, Theory of Ordinary differential equations, McGraw-Hill, New York 1955.
3. L. E. Elsgoltz, An estimate for the number of singular points of a dynamical system defined on a manifold, Amer. Math. Soc. Translation No. 68, 1952.
4. P. Hartman, On local homeomorphisms of Euclidean space, Proceedings of the Symposium on Ordinary Differential Equations, Mexico City, 1959.
5. S. Lefschetz, Differential Equations, Geometric Theory, New York 1957.