Unitary representations of the Lorentz group and description of particles with spin in the quasipotential approach

An expansion over unitary irreducible representations of the Lorentz group has been found lor the case of arbitrary spin in the spinor and helicity bases using the parametrisation suitable for the quasipotential approach. The addition theorem was also established for the transformation kernel, the use of which makes it possible to construct the local quasipotential in the relativistic configuration space.