| author: | Sebastian Schmitt Institut für Festkörperphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany |
Starting from a cumulant expansion in terms of the transfer between lattice sites in the presence of large local Coulomb matrix elements, Bethe-Salpeter equations are derived. Using only local particle-hole or particle-particle irreducible vertices, as in the dynamical mean field theory, general susceptibilities can be approximated. We focus on the analytic continuation of such equations to the real axis and discuss methods of resolution. In the case of the Hubbard model the transition to the regime of stable localized moments can be studied analytically.