Anderson's "Theorem" and Bogoliubov-de Gennes Equations for Surfaces and   Impurities
  In order to incorporate spatial inhomogeneity due to nonmagnetic impurities, Anderson [1] proposed a BCS-type theory in which single-particle states in such an inhomogeneous system are used. We examine Anderson's proposal, in comparison with the Bogoliubov-de Gennes equations, for the attractive Hubbard model on a system with surfaces and impurities.   [1] P. W. Anderson, J. Phys. Chem. Solids {\bf 11}, 26 (1959). 
K. Tanaka and F. Marsiglio