Dynamical Groups and Mass Formula

The homogeneous Lorentz group and the 4+1 de Sitter group are interpreted as the dynamical groups of a nonrelativistic and a relativistic "rotator," respectively. In an irreducible representation of the latter group we obtain for certain states the mass formula m²=m₀²+λ²j(j+1). The contraction of the dynamical groups [Euclidean group in three dimensions and Poincar\'e group, respectively] destroys the energy or mass spectrum and can be associated with the limit →0. The model of an elementary particle as a de Sitter "rotator" is discussed.