The \pn{Dulac} criterion is a classical method to rule out the existence of periodic solutions in planar differential equations. In this paper the applicability and therefore reversibility of this criterion is under consideration.
Bifurcation of periodic solutions from a ring configuration in the vortex and filament problems
Quantitative stability of certain families of periodic solutions in the Sitnikov problem
Braids, metallic ratios and periodic solutions of the $2n$-body problem
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