  In the electron-phonon model, the influence of nonmagnetic impurities on the
transition temperature of superconductors is revisited. Anderson's pairing
condition between time-reversed eigenstate pairs is derived from the physical
constraint of the Anomalous Green's function. After this pairing condition is
incorporated into the self-consistency equation, one may find that the
phonon-mediated interaction decreases exponentially by localization.
