Relativistic forces in Lagangian mechanics
  We give a general definition of \emph{relativistic force} in the context of Lagrangian mechanics. Once this is done we prove that the only relativistic forces which are linear on the velocities are those coming from differential 2-forms defined on the configuration space. In this sense, electromagnetic fields provide a mechanical system with the simplest type of relativistic forces. 
J. Mu\~noz D\'iaz
math-ph gr-qc math.MP physics.class-ph