Perturbation theory for the Green's functions of the Heisenberg model with single-ion anisotropy

Instead of solving the set of the $2S$ equations for Green function of ferromagnetic with the single-ion anisotropy, it is suggested to solve a single equation for the parametric Green function. The solution of this equation is used for exact calculation of ferremagnet magnetization of spin $S=1$ with one and two sublattices. The approximate solution is found for arbitrary value of the spin.