Dynamic spin susceptibility in the Hubbard model

By employing a new diagram technique developed for systems with strong correlations, the dynamic spin susceptibility for the Hubbard model is calculated. The single-site interaction is allowed for in the zero-order approximation, while electron hopping is taken into account by the evolutoin operator. We consider the second order of the perturbation theory and calculate the sum of the two-particle single-site ladder-type diagrams that are irreducible. Equations determinig the static susceptebility and spin wave spectrum are obtained.