Direct Proof Of Tree-Level Recursion Relation In Yang-Mills Theory
  Recently, by using the known structure of one-loop scattering amplitudes for gluons in Yang-Mills theory, a recursion relation for tree-level scattering amplitudes has been deduced. Here, we give a short and direct proof of this recursion relation based on properties of tree-level amplitudes only. 
Ruth Britto, Freddy Cachazo, Bo Feng, Edward Witten