Discontinuities of kinematic factors at zero masses and related problems

We expalin why some kinematical factors are discontinuous when one of the masses vanishes. We derive the kinematic factors for general reactions involving one massless particle. The method can be easily extended to more than one massless particle. We use the crossing relations to rederive and generalize in a simple way the kinematical constraints at s=0 recently obtained by Stack. We give a general expression for the determinant of the regularized crossing matrix and discuss its applications.