Relativistic shifts of eigenfrequencies in an ideal Penning trap

First-order perturbative calculation of the frequency-shifts caused by special relativity is performed for a charged particle confined in a Penning trap. The perturbed motion is approximated by the Jacobian elliptic functions which describe the periodic orbit repeating itself sinuously with a period that exceeds 2π. We find relativistic corrections to amplitudes of oscillating modes as well as shifts of eigenfrequencies which depend on amplitudes. Besides we find the relativistic contributions to modular angles. In the low-energy limit the deformed orbit simplifies to the well-known combination of axial oscillation and in-plane motion consisting of two circular modes. We compare the results with the model of relativistic frequency-shifts developed by J. Ketter et al. [1].