Journal of Statistical Physics, Vol. 96, Nos. 12, 1999. p. 439-445.
Two simple proofs of the result that a relativistic summational invariant psi is a linear combination of the momentum four-vector p^alpha are given by assuming that psi is a continuous and differentiable function of class C^2. The results can be extended to the case when psi is just assumed to be a generalized function.