Singularities of feynman amplitudes in the momentum space

Restrictions on the wave front of Feynman amplitudes in the momentum space are proved. It is established that the amplitude $F_\Gamma(q_1,\dots,q_k)$ corresponding to massless diagrams $\Gamma$ with two or three external lines are infinitely differentiable functions for $q_i^2\ne0$ ($i=1,2$ or $i=1,2,3$).