1. G. F. Duff, Limit cycles and vector fields,Ann. of Math., vol. 57 (1953), 15–31.
2. M. Urabe, Infinitesimal deformation of cycles,Jour. Sci. Hiroshima University, 1954, 37–53.
3. L. Marcus andA. Aeppli, Integral equivalence of vector fields on manifolds and bifurcation of differential systems,American Journal of Math., vol. 85 (1963), 633–654.
4. R. Sacker,Invariant surfaces and bifurcation of periodic solutions of ordinary differential equations, New York University, Courant Inst. Math. Sci. Tech. Rep. IMM-NYU 333, 1964.
5. M. C. Peixoto andM. Peixoto, Structural Stability in the plane with enlarged boundary conditions,Ann. Acad. Bras. Sci., vol. 81 (1959), 135–160.