Justification of partial-wave expansions of relativistic many-particle amplitudes

Conditions are found for the choice of relative variables in a system of $n$ asymptotically free particles under which the symmetry group of the $n$-particle phase space is isomorphic to the group $O(3n-3)$ of the movements of a sphere. The summation of events obtained from exclusive observations of reactions over the regions of this sphereis most convenient for the subsequent analysis.