Hamiltonian Systems: Generic Properties of Closed Orbits and Local Perturbations. F. TAKENS — 1970 Mathematische Annalen
The Minimal Number of Critical Points of a Function on a Compact Manifold and the Lusternik-Schnirelman Category. F. TAKENS Inventiones mathematicae
Cycles and measure of bifurcation sets for two-dimensional diffeomorphisms. J., Takens, F. Palis — 1985 Inventiones mathematicae