Distribution function for relativistic charges in the vicinity of cyclotron resonance

The function of charge distribution over the momentum projections in a preset field of a circularly polarized rf transverse electromagnetic wave propagating along a constant uniform magnetic field with phase velocity $u > c$ is obtained by solving the relativistic collisionless kinetic equation. The peculiarities in energy associated with the nonlinearity of the cyclotron resonance for various values of the phase velocity are analyzed in the limits of single-valuedness of the resonance curve. Some distribution functions are considered.