All-loop group-theory constraints for color-ordered SU(N) gauge-theory   amplitudes
  We derive constraints on the color-ordered amplitudes of the L-loop four-point function in SU(N) gauge theories that arise solely from the structure of the gauge group. These constraints generalize well-known group theory relations, such as U(1) decoupling identities, to all loop orders. 
Stephen G. Naculich
hep-th hep-ph