  The principal series of unitary representations of the Lorentz group has been
considered in the helicity basis. Decompositions of tensor products of the
spinspaces are studied in the framework of projective representations of the
symmetric group. Higher-spin Gel'fand-Yaglom equations are defined in the
helicity basis over an arbitrary representation space. Applications of
decomposable and indecomposable Gel'fand-Yaglom equations to particle physics
are discussed.
