On the analytic structure of Green's function for the Fano - Anderson   model
  We study analytic structure of the Green's function (GF) for the exactly solvable Fano-Anderson model. We analyze the GF poles, branch points and Riemann surface, and show how the Fermi's Golden Rule, valid in perturbative regime for not to large time, appears in this context. The knowledge of analytic structure of the GF in frequency representation opens opportunities for obtaining formulas for the GF in time representation alternative to the standard one using the spectral density. 
E. Kogan