Bifurcation from periodic solutions of central force problems in the three-dimensional space

The paper deals with electromagnetic perturbations of a central force problem of the form ddt(φ(x˙))=V′(|x|)x|x|+Eε(t,x)+x˙∧Bε(t,x),x∈R3∖{0}, where V:(0,+∞)→R is a smooth function, Eε and Bε are respectively the electric field and the magnetic field, smooth and periodic in time, ε∈R is a small parameter. The considered differential operator includes, as special cases, the classical one, φ(v)=mv, as well as that of special relativity, φ(v)=mv/1−|v|2/c2. We investigate whether non-circular periodic solutions of the unperturbed problem (i.e., with ε=0) can be continued into periodic solutions for ε≠0 small, both for the fixed-period problem and, if the perturbation is time-independent, for the fixed-energy problem. The proof is based on an abstract bifurcation theorem of variational nature, which is applied to suitable Hamiltonian action functionals. In checking the required non-degeneracy conditions we take advantage of the existence of partial action-angle coordinates as provided by the Mishchenko–Fomenko theorem for superintegrable systems. Physically relevant problems to which our results can be applied are homogeneous central force problems in classical mechanics and the Kepler problem in special relativity.