Auxiliary particles and Fumi's theorem

Fumi's theorem relates the ground state energy to phase shifts and hence th e scattering matrix. In turn a theorem associated with the auxiliary partic les relates the ground state energy to its self-energy, on shell and implie s that the latter can be written in terms of the scattering matrix. These r elationships are described along with the role of the infrared divergences. (C) 1999 Elsevier Science B.V. All rights reserved.