  We remove a self-interaction from the Galitskii-Feynman T-matrix
approximation. This correction has no effect in the normal state but makes the
theory applicable to the superconducting state. It is shown that identical
theory is obtained by removing nonphysical repeated collisions in the spirit of
the Fadeev-Watson-Lovelace multiple scattering expansion. Our correction does
not violate the two-particle symmetry of the T-matrix, therefore the present
theory is conserving in the Baym-Kadanoff sense. The theory is developed for
retarded interactions leading to the Eliashberg theory in the approximation of
a single pairing channel.
