(not quite) Theoretical Proof of the Lorentz Force Formula
  There is actually a mistake in this paper, but it is still a nice try worth a read. It is (not quite) proved that within the framework of Special Relativity, a force exerted on a \emph{classical particle} by a field must be of the form $\yv{E}+\yv{v}\times\yv{B}$, the Lorentz force form. The proof makes use of an action principle in which the action is the sum of a free particle part, and an interaction part. 
Yoav Kleinberger