A Microscopic Derivation of the Critical Magnetic Field in a   Superconductor
  The propagator for a noninteracting many electron system in a constant magnetic field in three space time dimensions is computed. This formula and the results of [FT1,2] are used to give a microscopic derivation of a BCS-equation with magnetic field. It is shown that this equation has no solution if the magnetic field is sufficiently large. Perturbation theory in the interaction around the magnetic field propagator is discussed. 
Detlef Lehmann (Swiss Federal Institute of Technology, Z\"urich;   present address: Institue for Advanced Study, Princeton)