The remarkable feature that a superconductor can display "gapless behavior" has been discovered by Abrikosov and Gor'kov (AG) in their study of the effect of magnetic impurities on superconductivity. To discuss the effects of external perturbations on superconductivity in the present formalism it is necessary to transcribe the different interaction Hamiltonians in matrix form. One of the important aspects of the microscopic theory of superconductivity is the appearance of the coherence factor in the various transition probabilities. There exist two coherence factors, which will call plus and minus coherence factors. In the description of superconducting properties, the concept of time-reversal symmetry is extremely useful. One of the fundamental properties of superconductivity is the perfect diamagnetic response to a weak external magnetic field. This fact shows clearly that not the existence of an energy gap but rather the existence of a pair correlation is the basic characterization of superconductivity.