On representations of the inhomogeneous de Sitter group and equations in   five-dimensional Minkowski space
  This paper is a continuation and elaboration of our brief notice quant-ph/0206057 (Nucl. Phys. B, 1968, 7, 79) where some approach to the variable-mass problem was proposed. Here we have found a definite realization of irreducible representations of the inhomogeneous group P(1,n), the group of translations and rotations in (1+n)-dimensional Minkowski space, in two classes (when P_0^2-P_k^2>0 and P_0^2-P_k^2<0). All P(1,n)-invariant equations of the Schrodinger-Foldy type are written down. Some equations of physical interpretation of the quantal scheme based on the inhomogeneous de Sitter group P(1,4) are discussed. The analysis of the Dirac and Kemmer-Duffin type equations in the P(1,4) scheme is carried out. A concrete realization of representations of the algebra P(1,4) connected with this equations, is obtained. The transformations of the Foldy-Wouthuysen type for this equations are found. It is shown that in the P(1,4) scheme of the Kemmer-Duffin type equation describes a fermion multiplet like the nucleon-antinucleon. 
Wilhelm I. Fushchych and Ivan Yu. Krivsky (Institute of Mathematics,   Kiev, Ukraine)