A relativistically invariant mass operator
  In Ukrain. J. Phys., 1967, V.12, N 5, p.741-746 it was shown how, for a given (discrete) mass spectrum of elementary or hypothetical particles, it was possible to construct a non-trivial algebra G containing a Poincare algebra P as a subalgebra so that the mass operator, defined throughout the space where one of the irreducible representations G is given, is self-conjugate and its spectrum coincides with the given mass spectrum. Such an algebra was constructed in explicit form for the nonrelativistic case, i.e., the generators were written for the algebra. However, the problem of how to assign the algebra G constructively and determine an explicit form of the mass operator in the relativistic case has remained unsolved. In the present work we present a solution of this problem, construct continuum analogs of the classical algebras U(N) and Sp(2N), and show that the problem of including the Poincare algebra can be formulated in the language of wave function equations. 
Wilhelm I. Fushchych