Gapless Hartree-Fock Resummation Scheme for the O(N) Model
  A modified selfconsistent Hartree-Fock approximation to the lambda*phi^4 theory with spontaneously broken O(N) symmetry is proposed. It preserves all the desirable features, like conservation laws and thermodynamic consistency, of the selfconsistent Dyson scheme generated from a 2PI functional, also known as the Phi-derivable scheme, while simultaneously respecting the Nambu-Goldstone theorem in the chiral-symmetry broken phase. Various approximate resummation schemes are discussed. 
Yu. B. Ivanov (1 and 2), F. Riek (2), J. Knoll (2) ((1) Kurchatov   Institute, (2) Darmstadt, GSI)