  Recently it has been shown that in gauge theories amplitudes to any
perturbation order can be obtained by glueing together simple three-point
on-shell amplitudes. These three-point amplitudes in turn are fixed by locality
and Lorentz invariance. This factorization into three-point on-shell amplitudes
follows from the BCFW recursion relations and the Feynman-tree theorem. In an
explicit example, that is, the four-gluon amplitude with all plus helicities,
we illustrate the method. In conventional calculation this amplitude
corresponds to one-loop box diagrams.
