Interaction of current carriers with local moments in a narrow band

One-electron Green's functions of a narrow-band paramagnetic insulator are calculated within the framework of the s-d exchange and Hubbard models with strong correlations. The agreement with the 1/z-perturbation theory is discussed for the approximations employed. It is demonstrated that the analytical properties of the retarded Green's functions are violated for simple approximations in the Hubbard model and the s-d model with a negative exchange parameter I (and, for self-consistent versions, at any sign of I). Numerical results for the corresponding densities of states are presented. (orig.)