Singularity Cancellation in Fermion Loops through Ward Identities
  Recently Neumayr and Metzner have shown that the connected N-point density- correlation functions of the two-dimensional and the one-dimensional Fermi gas at one-loop order generically vanish/are regular in the small momentum/small energy-momentum limits. Their result is based on an explicit analysis in the sequel of results of Feldman et al.[2]. In this note we use Ward identities to give a proof of the same fact - in a considerably shortened and simplified way - for any dimension of space. 
C.Kopper, J. Magnen (Centre de Physique Theorique, Ecole   Polytechnique)