  A recent article by Borisov et al. [Regular and Chaotic Dynamics 23.3 (2018):
339-354.] studies the motion of a rigid ball in a rotating-saddle trap. The
authors claim that they derive a new equation of motion from the Lagrangian
formalism, which is different from the one we obtained from the Newtonian
formalism in our recent work [American Journal of Physics 85.11 (2017):
821-829.]. We show here that these two equations of motion are the same. In
addition, besides the reduced spin frequency and the moment of inertia
coefficient, the stability condition given by the article is independent of the
ball radius---this result is incorrect. The mistake is due to the fact that the
center of mass and the contact point are not distinguished in the explicit
expression of the local normal vector.
