No-interaction theorem without Hamiltonian and Lagrangian formalism:   invariant momentum on null cones
  In a previous paper (G.Yoneda, Proc.R.Soc.London, A445,(1994),221), we proved the no-interaction theorem for four particles with the assumption that the (linear and angular) momentum on space-like planes is invariant. In this paper, we assume that the momentum on null cones is invariant and prove that there is no interaction for four particles. 
Gen Yoneda (Waseda U.) and Takesi Suzuki