Nearly-circular periodic solutions of perturbed relativistic Kepler problems: the fixed-period and the fixed-energy problems

The paper studies the existence of periodic solutions of a perturbed relativistic Kepler problem of the type \begin{equation*} \dfrac{$\mathrm{d}$}{$\mathrm{d}$t}\left(\frac{m\dot{x}}{\sqrt{1-|\dot{x}|^{2}/c^{2}}}\right) = -α\frac{x}{|x|^{3}} + \varepsilon , \nabla_{x} U(t,x), \qquad x \in $\mathbb{R}$^d\setminus{0}, \end{equation*} with $d=2$ or $d=3$, bifurcating, for $\epsilon$ small enough, from the set of circular solutions of the unperturbed system. Both the case of the fixed-period problem (assuming that $U$ is $T$-periodic in time) and the case of the fixed-energy problem (assuming that $U$ is independent of time) are considered.